Mac Mini: Time Machine fails, drives eject after upgrade

TROUBLE WITH TIME MACHINE AFTER MINI UPGRADE.

Hi. Had a Mini M2 Tahoe 512Gb paired with a 4Tb NVMe Internal drive for file storage. TM backups were always ok. Upgraded to a Mini M4 Tahoe 512Gb via Migration Assistant. Kept same NVMe drive. The system works fine.

The problem is with Time Machine. External drive properly formatted and readied for TM. It starts the backup but always aborts. No error code, simply a notice that TM couldn't make the security copy. Then a 2nd message comes up, Disk ejected improperly. Tried with 2nd external drive. Same results. Then tried to make a copy/paste from the internal and NVMe drives to an external one, but the process is often interrupted with error -8062. If I try to copy individual folders it is sometimes okay and sometimes the same error comes up. It is usually related to folder size. Does it have anything to do with the external drive size (tried with 4 and 5 Tb)? (When formatting for TM it tells me to better use a 8.99Tb or up drive.) Or is there a compatibility issue somewhere? Any advise re where to look?