Eliminating micro current flow
Seeing as you are certain it's the reset, and the reset is active low, you can re-create the reset. Before you do that you should verify that the reset when disconnected on the external drive pulls nicely high.

It should, because it is "sourcing power" to your expansion board, so it very likely has an internal pull-up causing all your troubles. If there is a pull-up inside the drive, you can just use two MOSFETs:







simulate this circuit &ndash; Schematic created using CircuitLab

They can be simple low power MOSFETs.



You can also disconnect the reset from its origin with board VCC with a little trick:







simulate this circuit

If the Board Vcc is high and the Board Reset is low, the MOSFET has a positive gate-source voltage and it will conduct and also allow the drive Reset to be pulled low by the reset signal.

If the Drive Reset is high, but Board Reset and Board VCC are low, the MOSFET has 0V gate-source voltage, and thus be turned off. The internal body diode in that case is in the blocking direction and in modern MOSFETs should not leak enough anymore.

If the Board Reset is high (regardless of the Board VCC) the body diode of the MOSFET will be put in the forward direction, and conduct, pulling the Drive Reset high, but of course with the voltage drop across the body diode subtracted. The very likely pull-up in the drive will undoubtedly take it the rest of the way.



If none of that "clicks" for you (pun intended), this certainly will:







simulate this circuit