External drive for Time Machine cannot be mounted, formatted or repaired I just upgraded my MacBook Air to 11.1.
I used to run regular backups on a WD 1TB disk using Time Machine. But for a couple of days, the backup has not completed, issuing a &quot;cannot find disk&quot; error. It just managed to complete a backup two days ago after the upgrade to 11.1.
I have erased the disk through Disk Utility, but I encounter the following errors when trying to further format or mount it. The disk's name is disk2s2.
Mounting on Disk Utility
Could not mount “disk2s2”. (com.apple.DiskManagement.disenter error -119930868.)

diskutil eraseVolume ExFAT MyName disk2s2
The target disk is in use by APFS as a Physical Store; use diskutil apfs deleteContainer

sudo diskutil eraseDisk HFS+ backupMac /dev/disk2s2
A volume was specified instead of a whole disk: /dev/disk2s2
Specify a whole disk, or instead use diskutil eraseVolume to erase a volume on a specific partition


diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI ⁨EFI⁩                     209.7 MB   disk0s1
   2:                 Apple_APFS ⁨Container disk1⁩         250.8 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume ⁨Macintoch HD - Data⁩     142.2 GB   disk1s1
   2:                APFS Volume ⁨Preboot⁩                 293.3 MB   disk1s2
   3:                APFS Volume ⁨Recovery⁩                613.6 MB   disk1s3
   4:                APFS Volume ⁨VM⁩                      5.4 GB     disk1s4
   5:                APFS Volume ⁨Macintoch HD⁩            22.7 GB    disk1s5
   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 22.7 GB    disk1s5s1

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI ⁨EFI⁩                     209.7 MB   disk2s1
   2:                 Apple_APFS ⁨⁩                        999.8 GB   disk2s2


sudo diskutil repairVolume /dev/disk2s2
Started file system repair on disk2s2
Error: -69564: Unable to find an APFS Container Reference

sudo diskutil verifyVolume /dev/disk2s2
Password:
Started file system verification on disk2s2
Error: -69564: Unable to find an APFS Container Reference  

I feel out of ideas.
This solution eventually worked for me,
I erased again the disk through Disk Utility, but giving it a different name.
I also erased the disk in the Time Machine preferences, and then assigned the &quot;new disk&quot;. It could then be found and a backup is currently on-going.