Time Machine Couldn't Compete the Backup

Yisang wrote:

2021 14" MBP running Sequoia 15.1.1 and trying to backup via Time Machine onto LaCie External Drive with plenty of space on it. Runs the backup for several hours as if nothing is wrong, but

then says it "did not finish backing up because some files were unavailable. Backups will resume when your Mac is unlocked."

How do I find out which files were "unavailable" and how could my Mac be locked if it was running? Thanks.

if there is file corruption, or disk corruption can throw this error.

you can try from TM logs for the last 1 hr and see if anything jimps out from the Terminal.app, copy & paste:

printf '\e[3J' && log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 1h | grep -F 'eMac' | grep -Fv 'etat' | awk -F']' '{print substr($0,1,19), $NF}'   

ref

If Time Machine on your Mac doesn’t back up certain files - Apple Support

Time Machine troubleshooting on Mac

Time Machine troubleshooting on Mac - Apple Support

Your Mac can be running while locked, e.g., when the screen saver is active but set to lock when it starts. When the Mac is locked, encrypted files are not able to be decrypted on-the-fly (which is the normal process) so they cannot be backed up by Time Machine.

Is this an initial backup or a routine/hourly backup? The initial one can take some time (especially if the drive is an HDD), in that case you can go to System Settings > Lock Screen and turn off the password requirement (you could also set a hot corner to prevent your Mac from sleeping).

Just to make sure, your external drive should be set up so that it's only used for a TM backup (or at least have a separate volume/partition on it for the backup).