Unable to unmount drive!
Open Terminal and try entering following:

df


which will give a listing of your mounted volumes similar to:

Filesystem    512-blocks       Used  Available Capacity  Mounted on
/dev/disk0s2  1951845952 1655141456  296192496    85%    /
devfs                477        477          0   100%    /dev
map -hosts             0          0          0   100%    /net
map auto_home          0          0          0   100%    /home
/dev/disk1s2  1952853344  754604936 1198248408    39%    /Volumes/Backup


depending on which drive you need to unmount you can then try:

sudo hdiutil eject -force /dev/disk1s2


which would force /Volumes/Backup to be unmounted.