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Unmounting disks in Ubuntu is a bit more challenging than it should be. Say you have an external drive mounted on /media/disk. Trying the standard

sudo umount /media/disk

will often result in an error such as

umount: /media/drive: device is busy.

It is advisable to check what files may be open on the drive with

lsof | grep /media/disk

If something important is still open on the disk, make sure to close the files or programs using the disk. If nothing else should be keeping the disk busy, you can usually achieve an unmount with

sudo umount -vl /mnt/external_disk

This performs a verbose, lazy unmount – i.e. Ubuntu unmounts the disk drive when the device is no longer busy, showing any errors in the unmount if there are any.

To confirm that the device has been properly unmounted, use

mount

This will list all the currently mounted devices – your drive should no longer be in the list. If your list is too long or you don’t feel like scanning it mount | grep /media/disk will return nothing if the device has been successfully unmounted.

In case of an unresponsive NFS mount, you can also try

sudo umount -vf /mnt/nameofnfsmount

The -f argument forces an unmount for unresponsive NFS and usually doesn’t help in other cases.

Although umount should perform this automatically, running sync after the unmount can make sure that the file system was properly synced. Happy unmounting!