My book and backup question

I've just bought a Western Digital My Book 1 tera byte usb drive which 
works well except for the purpose that I bought it for.

Mandriva's backup utility recognises the drive but I cannot allocate 
space for the backup, maximum space = 0. I had originally formatted the 
drive into two ext4 partitions with gparted and thinking that might be 
the problem I reformatted to ext3. The result is the same. I also tried 
a smaller partition, but again, with the same result.

I have another external drive (brand unknown) that I've used for years 
but it is now too small for the amount of backup data.

Can anyone offer a solution?

-- 
Regards,
Phil