/sys$common/syshlp/helplib.hlb Sys Parameters, TAPE_MVTIMEOUT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    TAPE_MVTIMEOUT is the time in seconds that a mount verification
    attempt continues on a given magnetic tape volume. If the mount
    verification does not recover the volume within that time, the
    I/O operations outstanding to the volume terminate abnormally.

    TAPE_MVTIMEOUT is a DYNAMIC parameter.