Fixing when SuperDuper! Smart Update doesn't appear to free up storage
 

This is an FYI for anyone else who hits this problem. Your mileage may vary.

I deleted about a terabyte of data completely off a 3 TB source volume, and used SuperDuper! Smart Update to re-clone to a slightly smaller backup, but afterwards the backup only had around 300 GB free. What gives?

The reason was that there was an old com.apple.asr snapshot hanging around on the backup volume. It was probably from when I used SuperDuper! erase-then-backup to clone this Big Sur backup drive to another drive. This snapshot may have been from an unsuccessful asr step, or one I interrupted. But for whatever reason, it was still hanging around.

Disk Utility in Big Sur doesn't list snapshots, so the procedure was:

1. diskutil apfs list to get the device ID of the problem Data volume.
   
2. diskutil apfs listSnapshots deviceid to list the snapshots on that volume. The result was:
   
   Code:

   ---

   Snapshot for disk8s2 (1 found)
|
+-- B87B74A0-4E24-4941-A276-15F1056135E9
    Name:        com.apple.asr.23515
    XID:        1841140
    Purgeable:  Yes
    NOTE:        This snapshot limits the minimum size of APFS Container disk8

   ---

   This gives the snapshot UUID (B87B74A0-4E24-4941-A276-15F1056135E9)
   
3. diskutil apfs deleteSnapshot deviceid -uuid snapshot_uuid
   
4. Unmount the volume
5. Mount it again

Unfortunately, 'asr' can often not clean up properly from its failures (we've reported bugs against this), although subsequent asr runs will usually delete leftover snapshots.