See the most recent Volume Shadowing for OpenVMS Software Product Description 27.29. Units must be the same type of disk and must have the same physical geometry. Compatible disks must have the same number of physical blocks. For example, an RF73 disk can shadow another RF73, but cannot shadow an RA72. The Files-11 structure prepares a volume to receive and store data so that the operating system can locate it easily.
Hardware write protection stops volume shadowing software from maintaining identical volumes.
As a result, using unsupported SCSI disks can cause members to be removed from the shadow set, if certain error conditions arise that cannot be corrected. You can shadow data disks and system disks. Thus, a system disk need not be a single point of failure for any system that boots from that disk. System disk shadowing becomes especially important for OpenVMS Cluster systems that use a common system disk from which multiple computers boot. A limit of 10,000 single member shadow sets is allowed on a standalone system or on an OpenVMS Cluster system.