Macintosh HD - Data is the volume that holds everything you can change on your Mac: your home folder, the apps you install, their settings and caches. Macintosh HD is its partner, a read-only volume that holds macOS itself. Together they form a volume group, and Finder shows them as one disk called Macintosh HD.
Both volumes sit in one APFS container with three helper volumes that macOS manages: Preboot, Recovery and VM. All five share the container's free space, so no volume has a fixed size, and none of them is wasted space you can delete.
When your Mac fills up, it's almost always the data volume that grew. The way to get space back is to remove files from it, never to erase or delete one of the volumes. The sections below explain what each volume holds, what Disk Utility's "Other Volumes" figure means, and where partitions and Boot Camp fit in.
Why your Mac has two Macintosh HD volumes
Until 2019, macOS and your files lived together on one volume. Anything with enough permission could change system files, by accident or on purpose.
When did Apple split the startup disk?
macOS Catalina, version 10.15, moved the system onto a volume of its own that nothing can write to. Your files moved to a second, writable volume, then named Macintosh HD - Data.
macOS Big Sur went further. Apple's security guide describes the signed system volume: the system volume is sealed with a cryptographic hash of every byte, and your Mac checks that seal as it starts up. If the check fails, startup stops and you're asked to reinstall macOS.
What is a volume group?
A volume group is a system volume and a data volume that belong together. Apple's Disk Utility guide says macOS installs on a volume group with one volume for the system files and one for data.
The pair are created, started up and erased together. Disk Utility shows the group as one item, Macintosh HD, with a small arrow beside it; click the arrow to see the volumes in the group.
Why does Finder show only one Macintosh HD?
macOS joins the two volumes with firmlinks: fixed links that make a folder on the data volume appear in place inside the system volume. Open Macintosh HD in Finder and you see Applications, Library, System and Users side by side, though they come from both volumes.
The list of firmlinks is short and fixed. It includes Applications, Library, Users, Volumes, private, opt and usr/local, plus a few folders inside System. macOS Tahoe 26.4 added one more, pkg, the first new firmlink since Catalina.
Why is my data volume called Data, not Macintosh HD - Data?
Both names are normal. Apple's documentation uses Macintosh HD - Data, and many Macs show that name. On others, the same volume is named just Data.
Which name you see depends on how macOS was installed on that Mac. The name doesn't change what the volume does, and there's no reason to rename it.
Which volume holds each top-level folder?
When you browse Macintosh HD, you're looking at both volumes at once. This table shows where each folder you see really lives.
| Folder at the top of Macintosh HD | Lives on | Can you change what's in it? |
|---|---|---|
Applications | Both: most of Apple's apps on the system volume, yours on the data volume | Your own apps, yes |
Library | The data volume | Partly; an administrator's password for some items |
System | The system volume, with some folders from the data volume | No |
Users | The data volume | Your own home folder, yes |
Hidden private, opt, usr/local | The data volume | macOS and tools you install manage them |
Hidden bin, sbin, usr | The system volume | No |
What lives on Macintosh HD, the system volume
The system volume holds macOS: the System folder, the apps that come with macOS and the command-line tools Apple ships. Apple's security guide says no process can write to it, not even Apple's own, outside a macOS update.
Why is the system volume read-only?
Because it's sealed. Your Mac doesn't even start up from the volume itself: since macOS Big Sur it starts up from a snapshot of it, a frozen copy that the seal covers. Any change, even one byte, would break the seal.
That's why the system volume doesn't need FileVault. Apple's guide explains that the seal already detects tampering, so FileVault encrypts the data volume, where your files are, and the system stays verifiable.
How big is the system volume?
On one Mac with Apple silicon running macOS 27.0, it takes 14.1 GB. It changes only when macOS updates, and it doesn't grow as you use your Mac, because nothing can write to it.
macOS keeps more of itself elsewhere. Large shared parts of the system now live on the Preboot volume, and features such as Apple Intelligence, voices and dictionaries download to the data volume as you need them.
Can you delete files from Macintosh HD to free space?
Not from the system volume. You can't change it, and it isn't what fills a Mac anyway. Folders such as System and the hidden bin and usr are macOS's own, so leave them alone even when you can see them.
What you can change, when you open Macintosh HD in Finder, is on the data volume underneath: your apps, the Library folder's contents and your home folder. That's where all the space you can recover lives.
What is the com.apple.os.update snapshot?
It's the sealed snapshot your Mac starts up from. Each macOS update seals a new snapshot of the system volume, with a name that starts with com.apple.os.update, and your Mac starts up from that one.
It's part of how updates work, not a backup taking extra space. If an update can't finish, Apple's guide says the old system version can be restored without reinstalling. Time Machine's local snapshots are different: they're on the data volume, and they do hold space.
What lives on Macintosh HD - Data
Everything that changes is on the data volume. Apple's security guide lists it: your files, apps you install, frameworks and background services from other developers, and the folders you can write to, such as Applications, Library and Users.
Your home folder, apps and their data
Your home folder, with Desktop, Documents, Downloads, Pictures and Music, is here. So are the apps you install, and every folder in your Library, from caches and logs to Application Support, Containers and Mail.
Other accounts on the Mac keep their home folders here too. So does macOS's own changeable data, such as its temporary files, its logs and its records of what's installed.
Hidden folders at the top of the data volume
A few folders sit at the very top of the data volume, out of sight. macOS creates and maintains them, and they can take real space.
- .Spotlight-V100 holds the Spotlight index. It's usually small, but it can grow when indexing goes wrong, and rebuilding it in System Settings replaces it.
- .DocumentRevisions-V100 holds the earlier versions of documents that apps keep when you save. You manage them in the app with File › Revert To › Browse All Versions.
Neither is for you to clear in Finder. Both belong to macOS, and only macOS changes them.
Time Machine's local snapshots live here too
When Time Machine backs up automatically, it keeps snapshots of the data volume on the volume itself, about every hour, for 24 hours. A snapshot holds on to files you've deleted or changed since it was taken, and no folder shows that space.
Your Mac counts that space as available, and Time Machine removes snapshots as they age or when space is needed. The guide to Time Machine's local snapshots explains how to remove them sooner.
Why FileVault encrypts the data volume
FileVault protects what's private: your files and settings on the data volume. The system volume holds nothing personal, and its seal guards it against changes, so there's nothing private to encrypt.
Apple's guide adds a rule that follows from this: you can't turn off the seal while FileVault is on, because protection has to be consistent across both volumes of the group.
Why disk tools sometimes count your files twice
The data volume can be reached by two routes: through the firmlinks from the top of Macintosh HD, and directly in a hidden folder at the path /. Both lead to the same files.
A tool that follows both routes counts everything twice and can report more used space than the disk holds. If an app shows a total larger than your drive, that's a likely reason, not a hidden copy of your files.
Preboot, Recovery and VM: the hidden volumes
Apple's security guide says a container that starts up a Mac needs at least five volumes. Three of them are hidden: Preboot, VM and Recovery. All three are unencrypted; the swap files macOS keeps in VM are encrypted on their own.
What is the Preboot volume?
Preboot holds the data your Mac needs to start up each system volume in the container. If you have two versions of macOS in one container, they share it.
Since macOS Ventura, Preboot also holds cryptexes: sealed disk images with parts of macOS that Apple can update separately. They include Safari and the large shared caches of system libraries that apps load. On Apple silicon those caches cover Intel apps too, for Rosetta.
Why is the Preboot volume so large?
Mostly because of those cryptexes. Howard Oakley's Eclectic Light Company measured Preboot at just under 1 GB of startup files under macOS Monterey and more than 17 GB once cryptexes arrived in Ventura.
On one Mac with Apple silicon running macOS 27.0, Preboot took 21.7 GB. That's normal, and only macOS changes it. It grows and shrinks with updates, not with anything you do.
What is the Recovery volume?
The Recovery volume holds macOS Recovery, the built-in system you start up into to repair a disk, reinstall macOS or restore from a backup. Apple's guide says it must be readable without unlocking a system volume, so it works even when the data volume is locked.
On a Mac with Apple silicon, each macOS installation has its own paired recovery system, since macOS Monterey 12.0.1. You start it by holding the power button as your Mac turns on. On an Intel-based Mac, it's Command-R at startup, and Internet Recovery takes over if the local copy is damaged.
What is the VM volume?
VM is where macOS keeps virtual memory: the encrypted swap files it writes when apps need more memory than your Mac has, and the sleep image it can save before a long sleep. It has no fixed size; it grows and shrinks with memory use.
Restarting clears the swap files, and the rest is macOS's to manage. The post on swap files and the sleep image explains why they can take tens of gigabytes.
What are Update, xarts, iSCPreboot and Hardware?
If you look inside /, you'll see folders with these names beside Data, Preboot, Recovery and VM. They're places macOS mounts small volumes it uses during updates and, on Apple silicon, for startup security. None of them hold your files.
Macs with Apple silicon also have two small hidden containers outside the main one. The first, about half a gigabyte, holds startup security settings. The second holds a fallback copy of macOS Recovery, which Apple's security guide says your Mac tries if the paired one fails.
Can you delete Preboot, Recovery or VM?
No. Your Mac needs Preboot to start up, Recovery to repair itself and VM for memory. Deleting any of them can leave a Mac that won't start up or can't be repaired.
They're also not where unexpected space goes. On a Mac that's filling up, the data volume is what grew, so look there.
| Volume | What it holds | Size on one Mac running macOS 27 | Who manages it |
|---|---|---|---|
| Macintosh HD | macOS, sealed and read-only | 14.1 GB | macOS updates |
| Macintosh HD - Data or Data | Your files, apps, settings, caches, snapshots | 412.9 GB | You and your apps |
| Preboot | Startup files and cryptexes | 21.7 GB | macOS |
| Recovery | macOS Recovery for this installation | 3.1 GB | macOS |
| VM | Swap files and the sleep image | 20.4 GB | macOS, by memory use |
| Hidden containers on Apple silicon | Startup security settings, fallback Recovery | 0.5 GB and 5.4 GB | macOS |
- Data volume412.9 GB
- Preboot21.7 GB · Managed by macOS
- VM20.4 GB · Managed by macOS
- Macintosh HD system volume14.1 GB · Managed by macOS
- Fallback Recovery container5.4 GB · Managed by macOS
- Recovery3.1 GB · Managed by macOS
- Startup security container0.5 GB · Managed by macOS
Measured on one Mac with Apple silicon running macOS 27.0 in October 2026, with a 494.4 GB container. Preboot and VM vary with the Mac, its memory use and its macOS version, and the data volume with what you keep.
Other volumes in container: what Disk Utility shows
Disk Utility is where the volumes become visible. It's in the Utilities folder: in Finder, choose Go › Utilities, then open Disk Utility.
How do you see all your volumes in Disk Utility?
Disk Utility opens in a simple view that lists only volumes. Choose View › Show All Devices to see the whole structure: the physical drive, the container on it, and the volumes inside the container. Apple's guide says some tasks, such as erasing a whole drive, are only available in this view.
Select the container to see its total size and free space. Select a volume to see what it uses, then click the Info button in the toolbar for more detail, including its purgeable and free space. The post on Disk Utility's storage figures goes through each one.
What does Other Volumes mean in Disk Utility?
Select a volume, and the bar at the top shows three figures: what that volume uses, what every other volume in the same container uses, and what's free. Storage settings shows the same idea with a crosshatched area for used space, other volumes included.
So the figure depends on what you select. Select Macintosh HD, and the data volume with all your files can count among the "other" volumes, which is why the figure can look alarmingly large.
Why is Other Volumes in container so large?
On most Macs, the answer is your own data. Your files, apps, caches and Time Machine's local snapshots are all on the data volume, and Preboot and VM add their share. A large figure is the same space Storage settings shows as used, seen from a different angle.
Occasionally there's another cause: a volume or a whole volume group left over from an earlier installation, a test of a new macOS version, or a reinstall that didn't go as planned. Disk Utility's full view lists every volume in the container, so an unexpected one stands out.
What should you do about a volume you don't recognize?
Don't delete it straight away. Note its name and size, and check whether it's a second macOS you or someone else installed: System Settings › General › Startup Disk lists every system your Mac can start up from.
Back up first, then delete it only once you're sure nothing on it is needed. Apple's guide says deleting a volume permanently erases all its data, and that Delete is dimmed for a volume that's in use.
How does this compare with Storage settings?
Choose Apple menu › System Settings, click General, then Storage. The main bar sorts used space by kind, not by volume. Click All Volumes… for a bar per volume and container on your internal disk; the steps are the same in macOS 27 Golden Gate and macOS 26 Tahoe.
The two views answer different questions. Storage settings tells you what kind of data fills the disk; Disk Utility tells you which volume it's on. The post on how to check storage on a Mac goes through each view's numbers.
| Where you look | What it shows | Best for |
|---|---|---|
| Finder, Macintosh HD | Both volumes as one disk | Browsing your files and apps |
| Storage settings | Used space by category, and All Volumes | Finding what kind of data is large |
| Disk Utility, Show Only Volumes | Volumes and the volume group | Used, other volumes and free at a glance |
| Disk Utility, Show All Devices | Drive, container and every volume | Seeing extra volumes and partitions |
| Disk Utility, Show APFS Snapshots | Snapshots of the selected volume | Time Machine's local snapshots |
How APFS shares space between volumes
Older Mac disks were divided into partitions of fixed sizes. APFS works differently: one container holds several volumes, and each takes space as it needs it.
Free space belongs to the container
Apple's Disk Utility guide puts it simply: when one APFS container has several volumes, its free space is shared and allocated to each volume as needed. No volume has space set aside that the others can't use.
That's why every volume on your startup disk shows the same free space. It's also why a growing VM volume or a larger Preboot after an update reduces what's free for your files, even though you didn't add anything.
Reserve and quota sizes
When you add a volume, Disk Utility's Size Options let you set two limits. A reserve size keeps an amount of space available for that volume. A quota size caps how much the volume can take.
They're useful for a volume you add yourself, such as one for a test macOS, or one for a project you want to keep from filling the disk.
Where does the space go when you delete a volume?
Back to the container, at once. The container's free space grows by whatever the volume used, and every other volume can use it.
Deleting files works the same way at a smaller scale: the space returns to the container once the files are gone from the Trash and no local snapshot still holds them.
Partitions, extra volumes and Boot Camp
A partition divides a drive into separate parts with fixed sizes. Each APFS container is itself a partition, and your startup disk normally has one main container, plus the small hidden ones on Apple silicon.
Should you partition a Mac's disk?
Usually not. Apple's guide says that with APFS you shouldn't partition your disk in most cases; add an APFS volume to the container instead. A volume shares free space, while a partition's size is fixed and hard to change later.
A partition makes sense when you need a different format, such as exFAT for a drive you share with Windows, or for Windows itself through Boot Camp. Back up before you change partitions on any drive.
| APFS volume | Partition | |
|---|---|---|
| Size | Grows and shrinks; shares the container's free space | Fixed when you create it |
| Format | One of the APFS formats | Any format Disk Utility offers |
| Typical use | A second macOS, a separate area for projects | Windows with Boot Camp, an exFAT area |
| Removing it | Delete Volume in Disk Utility | Disk Utility, or Boot Camp Assistant for Windows |
| Space after removing | Returns to the container at once | Can be added to the partition before it |
How do you run a second version of macOS?
Apple's article on using more than one version of macOS describes it: in Disk Utility, select your startup volume, choose Edit › Add APFS Volume, name the volume and click Add. Then run the full macOS installer and choose the new volume when it asks where to install.
The installer creates a second volume group in the same container: its own system volume and data volume. Preboot, Recovery and VM are shared, not duplicated, so the cost is the second system and whatever you keep on its data volume.
How do you remove a second macOS?
Start up from the macOS you're keeping, from System Settings › General › Startup Disk. Back up anything you need from the other one first.
Then open Disk Utility, select the volume you're removing and click the delete volume button. When it's part of a volume group, Disk Utility asks whether to delete the whole group; Apple's article says to delete the group, so both halves go.
Boot Camp on Intel-based Macs
Boot Camp Assistant installs Windows on an Intel-based Mac in a partition of its own named BOOTCAMP. Apple asks for at least 64 GB of free space, and says 128 GB gives the best experience.
Choose its size carefully: Apple says you can't change it later. While it exists, that space belongs to Windows, and macOS can't use it for your files.
How do you remove a Boot Camp partition?
Only with Boot Camp Assistant. Apple says plainly not to use any other utility to remove Windows or a partition Boot Camp created, Disk Utility included.
Back up what you need from Windows first, because removing it erases the partition. Start up in macOS, quit your apps, open Boot Camp Assistant, click Continue, choose to remove Windows if it asks, then click Restore. The space returns to your Mac's container.
What about Windows on a Mac with Apple silicon?
Boot Camp is for Intel-based Macs only, and Apple's Boot Camp article lists only Intel models. On a Mac with Apple silicon, Windows runs in a virtual machine app instead.
A virtual machine keeps its disk as a large file on your data volume, not in a partition. That makes it easy to remove, and easy to forget: virtual machine disk space on a Mac explains how those files grow and how to reclaim them.
Mistakes to avoid with your startup volumes
Most trouble with these volumes comes from treating them as folders to clean. They aren't, and Disk Utility's tools for them erase rather than tidy.
Which one do you erase when you reset a Mac?
On a Mac with Apple silicon or an Apple T2 Security Chip, use Erase All Content and Settings, which needs macOS Monterey or later. Choose Apple menu › System Settings, click General, click Transfer or Reset, then click Erase All Content and Settings.
If you erase in Disk Utility from macOS Recovery, Apple's article for Macs with Apple silicon says to select Macintosh HD and click Erase Volume Group when Disk Utility offers it. That erases both halves together.
Don't erase the data volume to free space
Erasing Macintosh HD - Data removes every account, file, app and setting on the Mac. It's what a full reset does, not a way to clean up, and it can't be undone.
If you need space, find what's large on the data volume and remove that. Even a very full Mac usually has a handful of large items, such as backups, videos, virtual machines or developer files, that account for most of it.
Don't remove volumes you don't recognize without checking
A volume you didn't create isn't necessarily spare. Preboot, Recovery and VM are always there, a second volume group may be another macOS someone uses, and Xcode's simulator runtimes appear in Disk Utility as disk images with volumes of their own.
Check its name, its role and whether it's a startup disk. When in doubt, back up and leave it.
Prefer a volume to a new partition
Resizing your startup container to add a partition moves data around and can take a long time; Disk Utility warns you before it starts. Apple's guide asks you to back up your data before you create partitions on any device.
Free some space first, back up, and prefer an APFS volume to a new partition. A volume needs no resizing.
Don't follow Terminal recipes for volumes
Some web pages suggest commands to delete, resize or mount volumes. Disk Utility and Boot Camp Assistant do everything a person normally needs, with confirmations and warnings, and a mistyped command can erase the wrong volume in an instant.
How to free up space on Macintosh HD - Data
Everything you can recover is on the data volume, so this is where cleanup happens. The volume structure explains the numbers; it doesn't hide extra space.
Start with what's largest
In System Settings › General › Storage, the categories with an information button list their largest items. Typical large items on the data volume are iPhone backups, videos, old downloads, Messages attachments, apps you don't use, and on a developer's Mac, build files and local AI models.
System Data has no button. Its largest parts are usually Time Machine's local snapshots, caches, swap and app data, which live on the data volume and in the VM volume.
What macOS frees by itself
Time Machine removes local snapshots as they age or when space is needed. Restarting clears swap files on the VM volume. Purgeable space, such as iCloud files macOS can download again, is freed when something needs the room.
That's why the free space in Disk Utility, the available space in Finder and the figure in Storage settings can differ by tens of gigabytes without anything being wrong.
Keep enough free space for updates
A macOS update needs room to download and prepare the new system before your Mac switches to it. All of that comes out of the container's shared free space, so a nearly full data volume can block an update, even though the system volume itself barely changes size.
How the app does it
Wombit shows what's using your storage, explains every item in plain language, and clears it safely: Trash first, undoable and logged. It scans the whole disk or any folder, reading only names, sizes and dates, and file names and paths never leave your Mac.
Its space meter, in the app and the menu bar, shows what's available and how much of that is really free. Hard links and APFS clones count once, the way macOS counts space, and folders macOS didn't let it read are listed, not skipped.
System Data, explained splits that one number into named buckets, such as local snapshots, caches, logs and swap, and shows how much it couldn't explain. Local snapshots, swap, the Spotlight index and document versions are Managed by macOS: explained, handled through macOS's own mechanism, never moved.
Hourly snapshots of your startup disk that Time Machine keeps on the disk itself, so you can restore files even without your backup disk.
Managed by macOS · Runs tmutil for Time Machine's own snapshots, through the helper
/
private/ var/ vm Disk space macOS uses as extra memory when RAM runs low, and a copy of memory it can save before a long sleep.
Managed by macOS · Explained only: never removed
/
System/ Volumes/ Data/ .Spotlight-V100 The index Spotlight keeps for each disk so it can find files, and what's inside them, quickly.
Managed by macOS · Explained only: the app shows you how
/
System/ Volumes/ Data/ .DocumentRevisions-V100 Earlier versions of documents that apps like Pages, TextEdit and Preview save as you work, so you can go back to them.
Managed by macOS · Explained only: the app shows you how
The analyzer, the map, the explanations, the space meter and System Data explained are free. Cleanup, Time Machine snapshots and items that need an administrator are PRO: one payment of $4.99 with lifetime access. Every Pro tool's review is free to open, so you see what would go and why before you buy.
Frequently asked questions
What is Macintosh HD - Data on my Mac?
It's the writable half of your startup disk. It holds your home folder, the apps you install, their settings and caches, and everything else that changes. Macintosh HD, its partner, holds macOS and can't be changed, and Finder shows the two as one disk.
Can I delete Macintosh HD - Data to free up space?
No. Deleting or erasing it removes every account, file, app and setting on the Mac, and it can't be undone. To free space, remove large files, apps or backups from it instead.
Should I erase Macintosh HD or Macintosh HD - Data when resetting my Mac?
Both, together. On a Mac with Apple silicon or a T2 chip, use Erase All Content and Settings in System Settings › General › Transfer or Reset. In Disk Utility from macOS Recovery, select Macintosh HD and click Erase Volume Group when it's offered, so the pair stay matched.
How do I get rid of other volumes in container on a Mac?
Usually you can't, because most of that figure is your own data volume, plus Preboot and VM, which macOS needs. Free space by removing large files from the data volume. Delete a volume only if Disk Utility shows an extra one you've confirmed nobody uses, and back up first.
Why is the Preboot volume so big on my Mac?
Since macOS Ventura, Preboot holds cryptexes: sealed disk images with Safari and large shared parts of macOS that Apple can update separately. Measurements have found more than 17 GB under Ventura and over 21 GB on a Mac running macOS 27. Only macOS changes it, with updates, not with your files.
Is it safe to delete the VM volume?
No. It holds the swap files and sleep image macOS uses as extra memory, and your Mac needs it. Restarting clears the swap files, and quitting apps that use a lot of memory keeps it small.
Why are there two Macintosh HD icons in Disk Utility?
Disk Utility can show the volume group, named Macintosh HD, and the system and data volumes inside it. That's normal. If you see a second complete pair, it may be another macOS installation; check System Settings › General › Startup Disk before changing anything.
Sources
- Role of Apple File System (Apple Platform Security) · Apple
- Signed system volume security (Apple Platform Security) · Apple
- Add, delete, or erase APFS volumes in Disk Utility on Mac · Apple
- Partition a physical disk in Disk Utility on Mac · Apple
- Install Windows 10 on your Mac with Boot Camp Assistant · Apple
- What is Macintosh HD now? · The Eclectic Light Company