time machine partition not mountable anymore

hello,

the tm partition on my wd my book studio II is not mountable. the partition contains a sparsebundle with the backup.
a new created partition does work.
i tried the repair disk option in the disk utility and force mount, still no luck.
how can i fix this?
i need files from the backup. i would really like to install snow leopard soon ;-)

regards

 

does anybody know a way to repair the partition?