You may want to consider reformatting the drive that you are using for TM but be aware that this will destroy whatever data is on that drive.
Once reformatted, go through the steps to configure the drive for TM
Had to replace my HD. Lost all Time Machine backup for last several months. Now, can't back up TM.
Msg says TM needs work space to store backups. Strange because I haven't done anything to increase lack of space since getting my new HD.
I have 2 external hard drives--1 with 500GB for TM backup. Both HDs are shown in the select disks box. And, I can select the 500GB external HD as "use for backup." I then go to "exclude these items from backup" and BOTH the external HDs are shown--in gray! I'd like to exclude my second External HD and get more space for my TM backup. I can hit the + sign and add more items to exclude, but the minus sign is gray and I can't exclude anything--like my 2nd External HD. Any suggestions?