Fijate los comentarios de este usuario que le sucede lo mismo que a ti.
I tried to do svmotion with full CLI and not the interactive way and I understood that if I want to place individual disks I have to specify every disk even if I don't want to move it from the actual location. In my situation I had a VM with:
C: disk in SRV_EXC_DiscoC datastore
D: disk in SRV_EXC_DiscoD datastore
and I had to move C: to sata_SRV_S5_DiscoC datastore
the problem was that I only specified the change for C: disk, but trying to svmotion in this way will move all unspecified disks to the location of the VMX file, in fact when I specified also D: disk in the command line, saying he had to stay in the same datastatore that it was, the whole thing went succesfully.
The bad thing of all this is that svmotion is not so well documented and the vmware support didn't give me any hints in 2 months even if I wrote them the exact commands I typed to do the svmotion.
http://communities.vmware.com/thread/185516?start=15&tstart=0
Evidentemente debes especificar tanto el C origen y destino (el nuevo datastore) y para el D, deberas espeficiarlo nuevamente que sea el datastore donde esta alocado actualmente.
Slds
Diego