Mark,

IIRC, for a dual drive you can use the command "D1=0".

In all my time with the C64 I've only seen a double-drive configuration a
few times. And although I did fork over the money for that "breadbox" C64
drive because I got fed up rather fast with the casettes (always had to
verfy the program - on a medium that was already slow - to be sure it would
"stick". And I learned that the hard way. :-( )

Once at a user group meeting a saw someone had a program that would allow
multiple copies without connecting a computer (just once to load that
program)

As you could upload-end-execute programs onto the drives themselves (which
is what I did to get those "subdirectories" I spoke of earlier) I can easily
imagine that.

You could even copy between units ...

Thats the only way I saw it done.

... leaving the computer free, although the I/O bus would be unavailable
to it.

I once or twice considered throwing something together that would
effectivily create two seperate busses (a couple of 74xx open-collector
driver chips would have done it), but as I never had the pleasure of having
more than one device for that bus I had no reason to build it. Oh well.

Regards,
Rudy Wieser