to commonly asked questions.

Backup drive says it's full.

Scotty I got a pop up message that my seagate back up hard drive says FULL.

What do it do now? Last backup will be this morning

In a backup scenerio, there are two size factors. The source and the target. Your internal computer drive is the source. Your external backup drive is the target. The source should ALWAYS be equal to or greater than the target. If the target is smaller then you can sometimes see that message. If there is other data on the drive in addition to the backups.backupdb folder (like some stuff someone drug over to that drive) then this could also not leave room for the full contents of the internal to backup.

Time machine's job is to fill the drive so this is normal for the external to be full. I would check the backup.backupsdb folder and scroll down to the bottom of the long list. Make a note of the date of the last backup. Go back tomorrow and do it again. If you have a new folder at the bottom of the list with a newer date, you're fine. If the dates of folders stopped several days ago or so, then it may be time to move to a larger drive. We could always also wipe that drive and just start the process over again but it's just nicer to just move to a newer, larger drive.

Apple says the same thing here.
This image is a theme.plist hack