The Internal port have the DF0 & DF1 signal.
The External port have the DF1,DF2 & DF3 signal.
About the memory, this is not an issue at all since all is done in realtime. There is no "precalculated" buffer here, so regarding the memory it can emulate 1,2 or more drives without needed extra internal memory.
I fact i already have a firmware version with the 2 drives emulation but
as i said before the only thing that prevent me to release this feature is that the drive "AS IS" doesn't offer a second drive select. You need to connect a wire from a the jumper block to JB and add a pull-up resistor on the pcb on this new extra line.
To keep things as simple as possible i prefer to delay the release of this new feature for the moment. But this will be released
BTW : About the piezzo mod -> Nice. I think that the modification can be less difficult with a mosfet. have you tried this ?