You would use disk utility and reformat the drive. This will erase everything. Make sure anything you want to keep is backed up elsewhere. That being said, if you are using Time Machine for your back up it should just erase the oldest backups to make way for the new so you would not need to buy a new back up drive.Fine. How do I erase it?
When I try by putting any file in the trash, I get a message that any file in the backup disk cannot be modified (or erased).
Am I missing something?
Right or control click on the icon of the drive. Select "Get Info". At the bottom of the window is a check box to ignore permissions. If it is greyed out you will need to click on the little lock and enter your password to unlock the volume before you check the box.What are those?