If you try to unmount a partition and get a message like this:
# umount /media/usbdisk/
umount: /media/usbdisk: device is busy
use the lsof command to find out what programs are using what files:
# lsof /media/usbdisk/
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
bash 6925 root cwd DIR 8,17 4096 1 /media/usbdisk/
xmms 6979 root cwd DIR 8,17 4096 1 /media/usbdisk/
This shows that the programs bash and xmms are using the device. For an even clearer picture, use the device name rather than the mountpoint:
# lsof /dev/sdb1
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
bash 6925 root cwd DIR 8,17 4096 1 /media/usbdisk
xmms 6979 root cwd DIR 8,17 4096 1 /media/usbdisk
xmms 6979 root 8r REG 8,17 2713101 377 /media/usbdisk/a.mp3
You either can wait until those processes exit or terminate them manually.
