Not entirely sure. Many times, when people get that code, they are using an ipod, phone, or other media source via a direct cable connection. And it seems many times, the issue is resolved by replacing the cable.
In your case, I doubt there is a cable being used. Now it could be a defective flash drive, but it "might" also be that it's formatted in something other than what the HU supports (usually FAT16 and FAT32).
It could also be that you have a dirty connection, no idea how old your unit is or how much dirt/dust is in the air or your car for that matter.