Manually remove TimeMachine backups!

These steps just saved me from having to reformat an external drive:

- Open Time Machine Disk
- Locate and open the folder Backups.backupdb
- Go to view by date and choose oldest on top
- Select the backup you want to delete
- Open Teminal and type sudo rm -rvf
- Drag and drop the old folders you want to delete in the Terminal Window
- Enter you user password at the prompt

Couple of warnings - use rm -rvf carefully, especially with sudo!
Dragging and dropping the correct file path is a good move, even if you're experienced.
From here.