Time Machine won't back up due to Macintosh HD - Data

iggyybell24 wrote:

I keep receiving the following alert "Time Machine could not back up the disk “Macintosh HD - Data” because a snapshot of the disk could not be created."

When I have looked into it, I think it's because the HD - Data drive has been used to install the BigSur update. Is there a way to reinstall the software and then I can use Time Machine again to continue my backups to an external drive?

My "My Book" is 6TB and the HD -Data is taking up 980.12GB.

Time Machine troubleshooting on Mac - Apple Support

If you are connect to a hub, try a direct connection to test.

If you see a - Data volume mounted in Finder, then you likely have an orphaned Data volume from a previous OS (or you have multiple boot volumes).

Open Disk Utility and select each - Data volume.

The current volume will have a home icon on the name and a Mount Point of /System/Volumes/Data.

The orphan(s) will have a Mount Point of /Volumes.

If the orphaned - Data volume is not part of another startup volume, you can remove it using the Remove Volume button (–).

While I have not seen a case where all of your data was not transferred to the current Data volume, you may want to ensure that everything in the old home folder on the orphan doesn't contain any files not in your current home folder.

Time Machine in Big Sur makes a snapshot of the drive, then copies that snapshot to the TM backup disk.

I don't know why, but it is not able to make a snapshot of the volume.

There seems to be enough free space (a snapshot doesn't take up much at all, though).

Try repairing the Data volume in Disk Utility.