Library /sys$common/syshlp/sysmanhelp.hlb IO, REBUILD, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The IO REBUILD command rebuilds the system's device configuration
    tables by reading and parsing the SYS$SYSTEM:SYS$USER_CONFIG.DAT
    and SYS$SYSTEM:SYS$CONFIG.DAT files.

    To debug modifications to the SYS$SYSTEM:SYS$USER_CONFIG.DAT
    file, you can use the IO REBUILD and IO AUTOCONFIGURE commands to
    load drivers without having to reboot. Once you load a driver for
    an adapter, however, you cannot reload it without rebooting the
    system.