Macintosh HD and Macintosh HD Data, why there are two volumes

Since Catalina the startup disk has been two volumes rather than one. Macintosh HD holds macOS and takes no changes. Macintosh HD Data holds everything you own. Finder joins them so convincingly that most people never learn there are two. Then they open Disk Utility one day and assume something has gone wrong.

Nothing has. What follows is what each one is for and how to tell an ordinary pair from a genuine mess.

What each volume holds

The second row matters more than it looks. The Data volume is not simply your files. Parts of macOS that have to change while the Mac runs live there too. That rules out treating it as an optional extra.

Read more: The hidden Library folder on a Mac and what lives in it.

Why the split exists

Apple fixes everything on the system volume at the moment it builds the release. Nothing running on the Mac alters it afterwards, including anything that arrives uninvited.

Before Catalina the two shared one writable volume. Anything with enough privilege could rewrite parts of macOS. Separating them removes the possibility rather than defending against it.

You are not running from Macintosh HD. Big Sur went further and sealed the system volume cryptographically. What starts up is a read-only snapshot of that volume rather than the volume itself. Snapshots also decide whether a deleted file can still be recovered. Even the copy in use sits one step away from anything that could change it.

Why the Data volume is the older one

This is the part that reads backwards. It explains the naming.

When a Mac upgraded to Catalina, the installer did not create a new volume for your files. It took the volume you already had, renamed it to Macintosh HD Data and changed its role. Then it created a fresh volume alongside for the operating system.

So the volume with Data in the name is the original startup disk, carrying everything from before the upgrade. The one named plainly is the newcomer. Anyone reasoning from the names alone gets the history exactly the wrong way round.

What holds the two together

Two volumes appearing as one takes a mechanism. Apple built one for the purpose.

Firmlinks join specific folders across the volume boundary. A path starting on the read-only volume continues onto the writable one with no visible transition. Applications, Users and Library all work this way, so the result behaves like a single file system while remaining two.

macOS also groups them. That is why Disk Utility indents one under the other. It is also why erasing one on its own tends to go wrong.

Why neither volume has a size

Look for the capacity of either one and you will not find a useful number.

APFS puts both volumes inside a container. Every volume in a container draws from the same free space. That container is the layer HFS+ never had, explained in full in the way the Mac file system is laid out. Nothing gets allocated in advance. The system volume takes what macOS needs and the Data volume takes the rest, with the boundary moving on its own.

Partitioning worked the other way. Each slice had a fixed size. Filling one meant repartitioning the lot. The words look similar and the behaviour is not.

When a third volume is a problem

One System and one Data volume is correct. More than that is worth looking at.

The right column comes from an incomplete erase before a reinstall. Erasing only the volume named Macintosh HD leaves the old Data volume in place. The installer then makes a fresh one beside it. The Mac still starts, which is why this goes unnoticed for months.

The answer is to work one level up. Turn on Show All Devices from Disk Utility's View menu, then erase the drive itself rather than a volume inside it. That clears the whole container in one action.

Relocated Items, the folder left behind

Some Macs carry a folder called Relocated Items, most commonly in the Shared folder. It is a leftover from the same change.

The Catalina upgrade had to move anything sitting in a location about to become read only. Rather than delete those files it collected them there, with a note explaining what happened.

Nothing has gone into it since that upgrade. A Mac bought recently never had one. A folder sitting untouched for years deserves the same review as anything else.