vDP backup only one disk

Hi,

i have a VM with 4 disk, the first is a normal vmdk with the system of about 40GB, the other 3 disk are RDM in virtual mode.

I have to backup ONLY the first disk, the one with the OS.

I try to make a backup with only this disk and it seems that all is ok, but when i try to restore it, vdp say that it cann't be restored
because there are no space on vmfs because he have registered somewhere that th VM have also other 3 disk very big.

How can i make a backu of ONLY the OS disk?

Thank!!!

     Enrico