If you have tried to erase the contents of your Time machine backup (external drive) and received the “Cannot unmount disk” error – no fear. I too have received the error and resolved it by plugging the external drive into a different mac and used the disk utility to erase it. Don’t forget to choose the Max OS Extended (Journaled) option for Time Machine.
Fix the "Cannot unmount disk" error when erasing Time Machine backup
Posted by win
on Thursday, April 17
