Just got a 550. Have a 700GB USB drive with music and photos, and the 550 recognizes it as soon as it is plugged into either front or rear USB slots. However, 2 different 8GB USB memory sticks are not recognized in either the front or back slot. They both show up fine on a PC. They have no folders, just an ISO file at the root level. Upgraded over the Internet to the latest firmware. Both USB sticks are NTFS format, and Checkdisk shows no problems.
Edit - tested a total of 6 USB sticks, not including the USB disk. 2 out of 6 were recognized. All fine on PC. Nothing stands out as a cause.