first thoughts:
Is this related to saving/loading a transponder list?
... saving video?
... loading firmware?
What version firmware are you running?
What did you do most recently to the box?
Have you power cycled, and factory reset the receiver?
Have you tried another USB thumb drive?
Did you format the USB drive on your computer? (most STBs read FAT only)
You might try loading through the serial port if the STB has one.