As Alexander2010 says, a Power Cable is one having a 12KOhm resistor connecting the GND and the ID pins. So maybe your problem is due to some current leak either in the cable or in the nuvi. Then try using other cables and/or cleaning both USB connectors. If it still doesn't help then I can try to make a firmware patcher that forces entering the flash drive mode when a Power Cable is connected. However, this wouldn't be comfortable in everyday use.