Open Carefully -- Contents Under Pressure said:

>One thing I'd also like to see in the ports is the ability to truly 
>dynamically verify devices.  What I mean is, say you have sd0 and sd1
>at units 0 and 1 respectively.  Now you plug in a tape drive (st0).
>Before you plug it in you say:
>	# mt -f /dev/nrst0 st
>And you get:
>	/dev/nrst0:  Device not configured
>[presumably FKA "No such device or address"]
>
>When you plug it in, you should be able to repeat the command and get:
>
>Brand INC-5411 4mm helical scan:
>   sense key(0x6)= unit attention   residual= 0   retries= 0
>   file no= 0   block no= 0
>   
>If the device is not responding, retry it N times and then mark the
>device as gone.  If the device comes back, mark it as present and
>keep going.

This is, at least partly, a hardware design problem.  I know that you can
do this on the RS6000 and perhaps a few others, but usually the SCSI bus
hardware on most systems will react to this by wedging.
-- 
Rick Kelly	rmk@rmkhome.com	    rmk@tencats.rmkhome.com
                http://tencats.rmkhome.com