Hi Patrick,
I had the same problem, and with Chris's help, I managed to solve it by changing the USB stick.
The interesting thing is that the pendrive I used on my old s975 did not work on the sx900.
I decided to test it with an old pendrive I had saved, only 4gb, and "Bingo", it worked!
Try changing the pendrive, borrow from one of your friends just for testing, and then you buy whatever works.
A hug my friend!
Enildo