"Macintosh SSD - Data" is mounting in addition to the regular volume

Plazmotech wrote:

So when I updated to Big Sur, I decided to do a fresh wipe. I launched disk utility in recovery mode and wiped the volume "Macintosh SSD" and "Macintosh SSD - Data".

Now they both are mounted at startup. So my desktop looks like this:

https://discussions.apple.com/content/attachment/7f053cc9-4f31-4cac-a472-11c84d3af532

Why is this happening? I can use disk utility to unmount "Macintosh SSD - Data", but on startup its just mounted again.

Thanks

Well from Disk Utility>View>Show All Devices It is here you see the parent drive— if you did not erase/reformat the parent drive you end up with the anomaly on your Desktop as you indicate.

I suspect you have two - Data volumes.

Open DU and and delete the extra Volume.

From Disk Utility.app you can see the mount point

One will be mounted at /System/Volumes/Data this is the one you want to keep,

The other will be mounted at /Volumes and you can simple use the “ -“ to delete it.

Plazmotech wrote:

Thanks! Deleting the Data volume worked. But there is a second anomaly: on both my iMac and my MBP 16" we have a volume called "com.apple.os.update-77...". Renaming this back to "Macintosh SSD" does nothing.

That is normal in Big Sur. Big Sur always boots from a snapshot

Spin the carrot if you do not want to see it.

From the Terminal.app you can see, example

diskutil apfs listSnapshots disk1s1s1

Snapshot for disk1s1s1 (1 found)

+-- 28512519-6A86-48A5-B0F5-62221042795D

    Name:        com.apple.os.update-779BDF1556C6F688504E24FB29C75AFFABFCB91E701806FFFF35235E19914F1E

    XID:         2381471

  Purgeable:   No

    NOTE:        This snapshot limits the minimum size of APFS Container disk1

[ *note your device number would be disk1s6s1 ] for the above example.

This is different then Time Machine snapshot(s) that you would see with:

 tmutil listlocalsnapshots