This is an old one but if anyone else googles it it might be good to have a response.
It should be possible to do the same mod on the A2000 as the A500 or A600, attack the CIA-B directly and connect the CIA _SEL0 to motherboard signal _SEL2. That would mean, disconnect pin13 and 15 connect 13 to where 15 was.
This will disable the original df0: leave df1: as it were and connect df2: which would be the external drive to where df0: was.
I'm not sure it will work without a df1: hooked up, there's a jumper on the motherboard to disable df1, don't remember what it's called right now.
The _SELx are found on pins 13-16 if you need to mix these up some more, perhaps you want to external drives as df0: and df1: then you need to move CIA pin13 and 14 to where 15 and 16 was attached. Or add a switch so you can switch back.
Two sockets, some wire and a switch is all you need if you don't want to mess with the original hardware. Then just plug this "selector" on the motherboard and the CIA in your selector.
