Time Machine

Time Machine will not automatically back up to external drive. If I unmount the drive, then remount it, I can get it to work for a short while, but the problem comes right back. ("Time Machine failed to back up to "X")

• Running OS 11.6.1 on M1 chip (desktop)
• External drive is new WD, formatted as AFPS
• Have cleared NVRAM (found as a solution in a different thread); no luck
• Have run "First Aid" from Disk Utility on external WD drive (found as solution in a different thread); received following error:

"Repairing file system.

Volume could not be unmounted.

Restoring the original state found as mounted.

The repair cannot be performed because one or more volumes are mounted. : (-69565)

Operation failed…"

...so I unmounted the drive and ran it again, only to get:

"Repairing file system.

Volume is already unmounted.

Performing fsck_apfs -y -x /dev/rdisk9s1

error: container /dev/rdisk9 is mounted.

File system check exit code is 65.

Restoring the original state found as unmounted.

File system verify or repair failed. : (-69845)

Operation failed…"

Hoping for a better solution than ejecting, disconnecting, re-connecting, then manually selecting "Back up now" every time I want to backup the system. Appreciate any help you might lend.