VMS Help BACKUP Command, Qualifiers, /FAST  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Command Qualifier

    Processes an input Files-11 disk using a fast file scan. The fast
    file scan reads the index file on the Files-11 disk specified
    by the input specifier and creates a table of files that match
    the qualifiers you specified. A fast file scan is most useful
    when the input specifier includes most files on the volume and
    file-selection qualifiers (such as those that pertain to date or
    owner) specify a relatively small set of the files named.

    To perform a fast file scan, you need write access to the
    INDEXF.SYS file on the input medium or the input medium must
    be write-locked.