Guessing there us a way from the >>> prompt to change a drive's volune, but I did not find a way to do it.
From the dead sergeant, you can select a boot device. Volume information -- label, structure, ownership, etc., etc. -- are manifestated in the operating system (VMS). If you had an issue with the device on SCSI id 2, you could have physically removed that drive. The device would then not appear in the device configuration and the MOUNT command would fail:
%MOUNT-F-NOSUCHDEV, no such device available
Assuming there is no ON ERROR in the startup procedures, your system should then boot through the system startup to the point where you can login. At that point, you can reconnect the device and issue:
$ MCR SYSGEN AUTOCONFIGURE ALL
Then, a:
$ SHOW DEVICE D
should show you your disk devices including the device at DKx200:.
one last thing...I only had one bootable drive, and I could only get to the $ prompt from the drive that was configured to mount and load some stuff from the ID=2 drive. What you suggest above would work, but would still have to have done what I did to get into minimal boot, etc. At least in VMS 5 minimal boot does not mount nor include any other drive than the boot drive in a $ show dev d. Bill