|External hard drives, external optical drives, and any other type of USB adapter that connects a hard drive or an optical drive to a computer, requires that the USB port that they connect to must be able to supply the full max 500ma per port the USB specs allow for.|
If you connect a USB device to a port in a multiport external USB hub that connects to only one USB port on the computer, ports in USB hubs DO NOT work with everything, even when the hub is a "powered" hub - has an external power supply connected to it. If that type of external hub is NOT "powered" the total amount of current that all the devices connected to the hub can draw can never be more than 500ma if you want all the devices plugged into it to (possibly) work properly.
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.
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.