Fuser

Ever needed to umount a device or file system or needed to umount your portable USB drive
but you can't as you keep getting a "Device is busy" error.

You cannot umount a file system that has open files, file handles, or if the file system is currently in use. not knowing what is using the device or what is keeping it busy can be extremely frustrating.
fuser to the rescue.
fuser will tell you what processes are using a file system and keeping it busy, fuser will also allow you to kill the processes that are preventing you from umounting the filesystem or device.
Lets say it is your usb memory stick on /dev/sda1 that you cannot umount.
Type
fuser -v /dev/sda1

will show you what and who is locking your device.
Then type
fuser -km /dev/sda1
to kill all the processes that are locking up and keeping your device busy.
then you will be able to umount your device without any errors.

fuser will also tell you what process or user is accesing a specific file.
Type fuser -v /filename eg
fuser -v /home/cgerada/filename.txt
and if you wanted to kill the process that is locking up the file, simply type
fuser -km /home/cgerada/filename.txt