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

       /AVAILABLE
       /NOAVAILABLE

    Controls whether the specified disk or magnetic tape is to be
    considered available. You must dismount the specified disk
    or magnetic tape before entering the SET DEVICE/[NO]AVAILABLE
    command. If you specify the /NOAVAILABLE qualifier, any attempt
    to allocate or mount the specified disk or magnetic tape is
    prevented. Devices are automatically set /AVAILABLE when brought
    online even if the device had been previously set /NOAVAILABLE.