/sys$common/syshlp/helplib.hlb SET, VOLUME, Qualifiers, /REBUILD  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /REBUILD[=FORCE]

    Recovers caching limits for a volume that was dismounted
    improperly. If a disk volume was dismounted improperly (such
    as during a system failure), and was then remounted with the
    MOUNT/NOREBUILD command, you can use SET VOLUME/REBUILD to
    recover the caching that was in effect at the time of the
    dismount. The FORCE option forces the disk to be rebuilt
    unconditionally, thus updating the free block count in the disk
    volume's lock value block.