Time Machine

In trying to use a new external drive for Time Machine backup, I got a message that Time Machine needs to erase a Seagate drive in order to use it for backups. I clicked OK to erase, which was performed, but that got an error message "There was an error preparing the Time Machine backup disk “Backup 6. Couldn't unmount disk”. I manually erased the disk but I still get the error message, and am stuck at this place. Time Machine won't permit the new drive to be used for back up. Suggestions? Thank you!