mac time machine started giving me errors yesterday. Have restarted several times, with errors. Have 330GB of HD left on backup. Get Time Machine couldn’t complete the backup to “Seagate Backup Plus Drive”.

How old is the seagate drive? The 2.5" drives in particular seem to be much less reliable after a couple of years.

Do a verify of the backup via Time Machine.. and a check of the disk via Disk Utility in the Mac.. Tell us your results.

Also open the console and type backup into the search and see if there are more details for the actual errors.

Tesserax has posted several times how to get that info so you actually have details of the failure.

Re: "Oldest backups are deleted when your disk becomes full"

IMHO just buy a new disk.. and start a fresh backup.

I would also quit using TM as your main backup and move to Carbon Copy Cloner for example.. which is going to be a lot more reliable. Plus it can create a bootable clone to the External drive.