How macOS actually works

The two volumes behind your startup disk

Updated 2026-10-30 · 4 min read

Your Mac's startup disk is really two volumes, one for macOS and one for your files. Finder joins them into a single disk, and Disk Utility shows both.

Why there are two

Since macOS Catalina, the startup disk has been split into two volumes. Macintosh HD is the read-only system volume that holds macOS itself. Macintosh HD - Data is the writable volume that holds your files, apps and settings. Apple's article on the read-only system volume says exactly this, and adds that in the Finder both volumes appear as a single Macintosh HD. Disk Utility is where you see them as two.

The split protects the system. Nothing you do in everyday use can overwrite the files macOS needs to start, because that volume can't be written to. Apple says you can no longer store files in it, or write to the root directory (/) from Terminal.

You never need to touch the Data volume to use your Mac. Don't erase it: that is where your documents, photos and apps live, and erasing it deletes them.

How the two volumes fit together

They aren't two separate slices of the disk. Both sit in one APFS container and share its free space, so there is no fixed size for either, and df -h reports the same total size for both. To see the pair, run these read-only commands:

mount | grep -E " on / | on /System/Volumes/Data "
diskutil apfs listVolumeGroups

On the Mac used for this guide, mount showed / as apfs, sealed, local, read-only, and /System/Volumes/Data as a separate apfs mount flagged root data. The second command listed one volume group with two members: (System) named Macintosh HD, and (Data) named plain Data.

So the name varies. Apple's article calls the volume Macintosh HD - Data, and that's the name people search for, but its stored name can be just Data, as it was on the Mac used for this guide (Apple silicon, macOS 15.6). The role in brackets is the reliable part: the volume marked (Data) holds your files.

Firmlinks: why Finder shows one disk

If the files are on a different volume, how does /Users/yourname still work? Through firmlinks, which join folders on the Data volume to the matching paths in the system view. macOS lists them in a small text file you can read:

cat /usr/share/firmlinks

On the Mac used for this guide it had 18 lines, among them /Applications, /Library, /Users, /Volumes, /private and /usr/local. Each line pairs a path in the visible tree with its real location inside the Data volume. The result is one tidy tree, even though the files sit on two volumes.

That is also why you'll sometimes see paths that begin /System/Volumes/Data/Users/.... It's the same file as /Users/..., just reached by its real address. Some tools and error messages print that longer form.

When Data seems to take up your space

Because Data holds everything you create and install, it's the volume that grows. The system volume stays roughly the size of macOS itself: diskutil reported about 11 GB for it on the Mac used for this guide, against 461 GB on Data. If Disk Utility shows Data as the big one, that is normal, and the two figures aren't added together against your disk. To work out what is filling it, see why Mac storage figures differ, and APFS explained for how volumes in a container share space.

"Macintosh HD - Data" isn't mounted

You might see the Data volume dimmed or marked not mounted, typically in Disk Utility in macOS Recovery. With FileVault on, the Data volume is encrypted and stays locked until someone enters a password. Disk Utility's message for that state begins "This disk is locked" and points you to Mount. Select the volume, click Mount, and enter the login password of a user on that Mac. If the right password doesn't open it, stop and get help. Don't erase it to make the message go away.

The sealed volume and SIP

On the Mac used for this guide, mount reports the system volume as sealed. Sealing is separate from System Integrity Protection, which adds its own limits on what even an administrator can change. Both are covered in System Integrity Protection.

Searching a disk that has two volumes

Everywhere skips /System/Volumes/Data on purpose, because it is the same set of files you already see through /Users, /Applications and the rest. Searching the real path as well would list every file twice. So each file shows up once, under the path you know.

Common questions

Why do I have Macintosh HD and Macintosh HD - Data on my Mac?

Since macOS Catalina the startup disk is two volumes. Macintosh HD is the read-only system volume, and Macintosh HD - Data holds your files, apps and settings. Finder shows them as one disk.

Can I delete Macintosh HD - Data?

No. It holds your files, apps and settings, so erasing it deletes them. To wipe a Mac before passing it on, use System Settings > General > Transfer or Reset > Erase All Content and Settings where your Mac offers it.

Is Macintosh HD - Data taking up all my space?

It holds nearly everything you store, so it is the larger volume. Both volumes share one APFS container's free space, so the totals aren't added together.

Why is Macintosh HD - Data not mounted?

With FileVault on, the volume stays locked in Recovery until you enter a password. Select it in Disk Utility, click Mount and enter your login password. Don't erase it to clear the message.