Map APFS volumes and mounts

Modern macOS uses APFS volume groups and mounts to present one familiar startup hierarchy. What looks like a single “Macintosh HD” is really several volumes inside one APFS container, all sharing the same pool of free space.

The split that matters for troubleshooting: the sealed system volume and writable data volume have different roles. The system volume holds macOS itself, is cryptographically sealed, and mounts read-only at /. Your files, apps, and settings live on the data volume, mounted at /System/Volumes/Data. Firmlinks stitch the two together so /Applications and /Users feel like one filesystem.

Inspect rather than modify

diskutil apfs list

Read the tree: the container (something like disk3), then each APFS volume with its role. You will see Macintosh HD marked as the system volume and Macintosh HD - Data in the same volume group. Note the volume identifiers (disk3s1, disk3s5) so you can match them elsewhere.

Now connect volumes to paths:

mount | head -5
df -h / /System/Volumes/Data

mount shows which device is mounted where, and that / is read-only. df -h reports capacity and free space. Both lines show the same available space because the volumes share the container.

Snapshots

APFS also keeps local snapshots, point-in-time views of the data volume that Time Machine and the update process create:

tmutil listlocalsnapshots /
# com.apple.TimeMachine.2026-08-02-221304.local

Snapshots explain the classic mystery of “I deleted 40 GB and got no space back”. The space is held by a snapshot and is released when the snapshot expires.

The mistake to avoid

Do not delete a volume or snapshot because its name looks duplicated; verify its role and backup state first. Macintosh HD - Data is not a duplicate of Macintosh HD. It is your entire user data. Deleting storage metadata is the one troubleshooting move you usually cannot undo.