Quite a few people are seeing a variety of USB problems - some of these might be related to insufficient voltage on USB devices due to drops along lead, connector and internal fuses on the Pi. There's a long discussion about these issues (192 posts) in the Feedback and Requests forum, under "Design flaw? Polyfuses for USB current limiting"
http://www.raspberrypi.org/phpBB3/viewt ... t=5830&e=0