Like many others who have posted here, I've had this problem for quite a long time, both with Big Sur and after upgrading to Monterrey. The strange this is that it only happens with one of my drives. When I tried to run a First Aid in Disk Utility (for the disk that keeps mysteriously 'ejecting' itself — no pun intended), the operation failed. Running First Aid on “Main Backup - White” (disk4s2) Checking file system and repairing if necessary and if possible. Volume was successfully unmounted. Performing fsck_apfs -y -x /dev/rdisk4s2 error: container /dev/rdisk4 is mounted. File system check exit code is 65. Restoring the original state found as mounted. File system verify or repair failed. : (-69845) Operation failed… No idea what is going on...