While trying to add additional shared disk resource to existing MS cluster in Virtual Server 2005, I noticed that on a shared SCSI controller the only selectable option when adding a disk is ID 0. If the controller already has a disk with ID0 (as in quorum in this example), it not possible to add any further shared disks to the shared bus.
e.g. The virtual machine has two SCSI adapters. The Shared controller is attached as Virtual SCSI adapter 2 (SCSI adapter ID: 7). On the Virtual Hard Disk Properties page, clicking the attachment drop down only lists ID 0 for SCSI 1. The non shared adapter has ID 0 through 7 available for selection.