Probably you misunderstood me just a tad there about what clearing nv would likely achieve. I didn't mean it would change anything in regard to a possible boot problem linked to your non-kosher usb setup, i was suggesting that if there is some corruption of nonvol memory which was preventing the unit updating from gupdate.gcd it maybe would correct by clearing nv. But of course you then need to actually re-attempt the upgrade which is what i was meaning by: 'your risk so your call'. Clearing nv in itself is virtually without risk and can only potentially benefit the unit.
That said, i do agree that it's best to leave it alone and don't risk the update. It may function fine until kingdom come as it is. Once you get used to updating the maps and files from the card it's quite easy. I have two like that as i said in the other thread, but fortunately i can update firmware np in them. Thanks for your patience, sorry we haven't been able to achieve very much for you. Cheers.