PBHGetVInfo can be used to access these fields. When a mounted volume is placed offline, vcbDrvNum is cleared and vcbDRefNum is set to the two's complement of the drive number. If vcbDrvNum is zero and vcbDRefNum is negative, you know that the volume is offline. When a volume is ejected, vcbDrvNum is cleared and vcbDRefNum is set to the positive drive number. If vcbDrvNum is zero and vcbDRefNum is positive, you know that the volume is ejected.
Ejection implies being offline.
It grew when we went from MFS to HFS, and it may grow again. It's safest to use calls like PBHGetVInfo to get the information that you need. Inc. All rights reserved.