There are some circumstances where you have to unmount a drive, but it is not letting you. Giving an error, similar to this:
umount: /mnt/backup: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
Generally, it is a bad idea to unmount it while it is being used, but there are circumstances when a process has crashed and you need to unmount. In those instances you can use something called "lazy" umount:
umount -l /mnt/backup
and the disk is now safe to remove.
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