Postmortem:
The missing mirror can be removed; its disk can be restored to basic
form; this transforms the remaining mirror to a basic system; the
basic volume can be made dynamic, so can the restored disk; finally
the restored disk can be made into a mirror and the original remaining
mirror can then be re-mirrored. All this is done within the disk
manager. Boot.ini can be edited a little if you want to be tidy (new
boot.ini shows duplicate entries).