What was the size of the USB sitck?
Stern recommends only using 256Mb or below USB stick.
I updated a Pirates a few years back using a 512Mb stick, and the machine had no sound at all after that, then once I used a 256Mb stick, it updated and worked fine.
This would suggest their code for reading the USB stick is quite limited, which may well be the case.
Finding a 256MB stick could be a problem these days, though.
Their code IS limited... and the Stern website says that you need to use an "approved" 128Mb+ USB stick (a flyer I have here states to not use one above 265Mb).
At the time I bought mine, they were already hard to come by. Might be best to try eBay, or the WTB section here to see if someone has an old one they no longer need.