Changing disks while the system is up

[John B. Nagle]

     First off, of course you don't do it while any file system on the
volume is mounted.
     There is a bit defined in the hardware interface between the drive
and controller for all SMD-type drives which is set whenever the pack is 
changed and remains set until cleared by the controller.  The idea is
that the software should thus be able to notice a pack change and take
care of any housekeeping required.
     In RP06 and RM05 type removables, this bit just sets a bit in one
of the device registers, which UNIX ignores.  But the newer drives or drivers
might do something with it.
     Another little-known feature of the SMD interface - you can stop the
drive under program control.  Generally not useful in the UNIX environment,
though.