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

    The SHADOW_SYS_WAIT parameter extends the time a booting system
    waits for all current members of a mounted shadowed system disk
    to become available to this node. The shadow set must already be
    mounted by at least one other cluster node for this parameter to
    take effect.

    The default value is 480 seconds. Change this parameter to a
    higher value if you want the system to wait more than the 480-
    second default for all members to join the shadow set. You can
    set the parameter value to 1 through 65,535 seconds.