So grab yourself a small piece of veroboard, solder an OR gate there (with appropriate decoupling cap), wire appropriately to Vcc and GND, wire 3 of the 4 gates to GND and use the 4th gate to implement
_SEL0 OR MTR0
_SEL0 is Gary pin 16
MTR0 is Gary pin 46
and wire the OR gate output to the _RDY line, which is on ODD CIA (U7) pin7.
This will provide the missing circuitry from the "switch" you bought so it fully swaps the drives.
I have made a boot selector myself, put it in my A500 (already had spare holes since my youth). Added it to be able to boot from a so called Gotek drive hooked up in an external housing
Clever solution, I suppose the OR:ed signal should only be connected to _RDY when the X:ing is active?
