Microcontroller GPIO status when switched off ? Can current flow from internally in microcontroller from one gpio to other gpios when switched off?
Firstly, isolating the grounds between the Nano and the MOSFETs is a recipe for disaster. The MOSFETs have a high impedance gate input and when you isolate the grounds that gate voltage will float to some value that is indeterminate. You could easily destroy the MOSFET with gate-source breakdown. As for a solution, consider this: -







simulate this circuit &ndash; Schematic created using CircuitLab

With no signal produced by the Nano, M1 will be turned off and M2 turned on by the gate being pulled-up to the load_power_rail. To turn off the load-switch (MOSFET M2), the nano needs to apply 3.3 volts to M1's gate relative to ground/0 volts.

Choose MOSFET M1 so that it is a logic level switching device.

If necessary (it usually is) use a reverse protection diode across the load terminals to prevent inductive spikes destroying M2. 

When using multiple channels of this circuit, all loads must share the same common ground/0 volts. 

If this is a problem then you will need to use opto-isolation instead of M1.