I bought an external drive several months ago but I haven't used it heavily until now. I discovered today that I forgot to exclude it from Time Machine, and so when I started adding tons of files to it today Time Machine complained that it didn't have enough space to back up.
I excluded the external drive, and it still won't backup. When I look in the Backup.backupdb file, I can see the folder for the external drive, but I can't remove it.
How can I recover that space so it can back up my main drive again?