This is something that isnt documented very well anywhere and personally i have totally destroyed a MK3 unit once by not realising this and unplugging the device before its ready.
The MK3/Mk4 bmw drive unit powers down fully and correctly approximately 1/2mins after the ignition has been turned off, if the power supply is cut or interrupted before hand then the devices ram corrupts.
To fully powerdown/make sure the device is ready to be unplugged:
- Turn off ignition get out and close door
- Go to the boot and open
- Look at the device the light should flash (this is when its writing and reading the memory)
- Wait roughly 1-2mins until the light is totally off
If no light is active then the device is fully shutdown