Two Macintosh HDs in Disk Utility: what the Data volume is, and which one is yours
Since macOS Catalina your disk is split into a sealed system volume and a Data volume with your files. Why there are two, and which to check for space.
Disk Utility shows Macintosh HD and Macintosh HD - Data, both apparently the same size. You have not been charged twice for storage, and you do not have a spare disk. It is one disk split in two since macOS Catalina.
What each one holds
| Volume | Holds | Can you change it? |
|---|---|---|
| Macintosh HD | macOS itself and built-in apps | No, it is sealed and read only |
| Macintosh HD - Data | Your files, apps you install, settings, caches | Yes, this is where your space goes |
Both live in one APFS container and share its free space. That is why each one shows the same capacity and the same free space. The used figures are what differ.
diskutil apfs list | grep -E "Name|Capacity Consumed"Why it is split
Keeping the system on a sealed, read-only volume stops malware and mistakes from changing macOS, and lets updates replace the system cleanly. Finder merges the two, so you see one Macintosh HD with your Users folder and Applications side by side. The joining is done by what Apple calls firmlinks.
Which one to check for space
The Data volume. The system volume is a fixed size, typically 10 to 15 GB, and the same on every Mac with that macOS version. Everything that grows is on Data:
df -h /System/Volumes/DataThe other small volumes, Preboot, Recovery and VM, are covered in Other Volumes in the container, and the system share in how much space macOS takes.
Related: Mac drive can't be modified.
Common questions
What is Macintosh HD - Data?
It is the volume that holds your files, apps and settings. Since macOS Catalina, the system lives on a separate sealed volume called Macintosh HD, and Finder shows the two as one.
Why do Macintosh HD and Macintosh HD - Data show the same size?
They share one APFS container and its free space, so both report the same capacity and free space. Only the used space differs.
Can I delete Macintosh HD - Data?
Only as part of erasing the whole Mac. It contains all your files and apps.