|Troubleshooting USB device problems including for flash drives, external drives, memory cards.|
See Response 1:
Check that out first.
Rarely, not all the ports on the back of a desktop case may be able to supply 500ma each.
(A flash drive or memory card reader does not require 500ma, but doing this may help.)
If you have a desktop computer, Note that I answered a Topic on this site where a guy had an external drive, which does require the full 500ma, connected to a port on the back of a desktop case - it would not work properly when a webcam was in the port next to it, but it worked fine when the webcam was unplugged. Ports on the back of a desktop case often have two ports connected to the same USB controller module that are ports one above the other - you could try connecting the cable to one of those and leaving the other un-used.