Date   : Tue, 17 Jan 1984003:48:00 est
From   : Schauble@MIT-MULTICS.ARPA
Subject: Function 37 - disk door open

Some disk drives (my Qume DT8's for example) have a "disk change" line
on the interface.  This is driven by a flip-flop that is set when the
door is open and reset when the drive is unselected while ready. If you
select the drive and find -ready, there is presently not a disk in it.
If you find it ready and disk change, someone changed the disk while you
weren't looking. 

I don't know about the existance of this feature on 5 inch disks. I do
know that the IO driver discriptions for MS/DOS document an entry point
for "is this disk changed since the last I/O operation" that can return
"yes", "no", or "I don't know". So I suspect that it occurs on 5 inch
disks also.