umount: /mnt/backup: device is busy

Could not umount my external USB hard disk. The last time this happened was about a year ago. The paper I used to write down the solution was not around. Ok I could not find the note. Therefore I had to search the net again to find the command to display who is still using the device.

I should start to write down this kind of solutions on my web site. This will result not only in one place to search but also accessible from work and friends.

This block could also be the right place to collect the answers during I try to find the solution. After the problem is solved and I know all steps needed I is quickly transfered onto my web site.

Oh - the answer is lsof.