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Find process that block unmounting a partition or CDROM

You want to unmount a CDROM or a partition and get

umount: /boot: device is busy?

Just follow these steps to find out who blocks the partition:

# lsof /boot 
COMMAND  PID USER   FD   TYPE DEVICE SIZE NODE NAME 
tail    8022 Sash    3r   REG    3,1 1183 2022 /boot/grub/grub.conf

This output shows us that we got a tail command accessing /boot/grub/grub.conf.

You can either kill the given process with its PID or search through your consoles and shut down the programm using the file.