Force unmount of an External Hard drive

Hi there,

I've got an external HD (Western Digital) that used to work fine with 2 partitions : Time Machine and Dl Temp.
For some reasons, I can't see the Time Machine drive anymore. The DL temp is mounted onto the desktop, but Time Machine is not. If I open the Disk Utility, I sometimes see the other partition as an unmounted partition and sometimes, I don't even see it in the Disk Utility.

The thing is : impossible to do anything with that : it seems that the system know that te-here is another partition, tries to mount it, but while it does not succed in the operation, the process get idle and block all other things.
For instance, it blocks the computer Shutdown (when I manually unplug the HD, the computer shut down fine...)
I tried to format/erase/repartition it, but I never succed since the system can't unmount the DL temp.
It tried to log as root : same ****.
I tried to force unmount it with the terminal (umount -f /Volumes/"DL Temp"), but the process freezes, and it never unmount it.
I can't even try to format it under a windows OS, since both partitions are "Mac OS journalisé" (i don't know the translation for that one ^^. Maybe "logged"?).
Do you have any idea how to force unmount the HD so that I can't format it?

Thanks.