You get this error when trying to umount a partition:
umount /mnt/sdb1/
umount: /mnt/sdb1: device is busy
umount: /mnt/sdb1: device is busy


At a root prompt:
Run fuser see man fuser for options
fuser -v /mnt/sdb1/
Output of fuser
USER        PID ACCESS COMMAND
/mnt/sdb1/:          root       5116 ..c.. nautilus

Kill the offending process (or just shut it down
kill 5116
Check again
fuser -v /mnt/sdb1/
Everything clear run umount
umount /mnt/sdb1/

If it still wont unmount maybe try
umount -f -l /mnt/sdb1