External Hard Drive backing up with Time Machine

Hi, I have had no problem backing up to my external drive until today. It says the backup drive is full. I looked and I deleted other backups. There is nothing on it. It shows on the disk utility that it has 1TB. So I thought I would try to reformat it. I went to the disk utility and clicked erase (after putting it on MAC OS Extended (Journaled) and GUID Partition Map) and this is the error message I get.

Erasing “Seagate BUP Slim Media” (disk2) and creating “Seagate”

Unmounting disk

The volume on disk2 couldn’t be unmounted because it is in use by process 483 (Finder)

Couldn’t open device. : (-69877)

Also the Seagate is greyed out in the exclude these items from backup. I can't delete it. Every time it tries to backup I get Time Machine couldn’t complete the backup to “Backup Plus”Time Machine could not back up the disk “Macintosh HD - Data” because it is nearly full

Delete some files on this disk and try again.

Can you help me so I can start backing up again.

Operation failed…