The DISMOUNT command dismounts a library disk and deassigns  the
 logical  names  associated  with  it.  If no device is specified
 then the device defined by LIB$SYSDEVICE is dismounted.

 Format:

      DISMOUNT [ddcu:]

Additional information available:

Parameters

 ddcu:

 In  the  device  name,  "dd"  specifies  the  device  type,  "c"
 designates  the  controller,  and "u", the unit number, uniquely
 identifies a device on a common controller.