I am away from home right now so Will be short and sweet. I have monkeyed around with my ng4 for months now. The unit works fine for radio, parking sensors, etc but provides a constant ‘please wait’ message when the traffic or nav buttons are pressed when I run the unit without a hard drive inserted. This is why I was suggesting that perhaps the hard drive was damaged.
Last I heard the dealership only replace these units, at a cost of over £2k, so a cheaper option could be to replace the hard drive and see if it solves the issue. A good replacement to the hard drive is a compact flash as the original hard drives are very difficult To source.
Replacing the HD with a a CF (using an IDE converter) is relatively straight forward and would cost in the region of £25, but does require you to take the unit out of the car, or at least partially, as there is a drawer on the left side providing access to the HD. Getting the CF in this way is not easy but I have managed it a few times using needle nose pliers. Once the CF is in run the firmware via the usb and it will create all the partitions. Then install the maps and you should be back in business. If not, then you are only £25 Down and a couple of hours wasted.