|An external hard drive (or an external optical drive) requires that the USB port (or TWO USB ports) you plug it into can supply 500ma (1/2 amp) of current. |
The lower down front ports on a desktop tower case are usually individually connected to one USB port connection on a header on the mboard, so those should be able to supply 500ma each. However, the wiring between the mboard header and the lower down front ports is sometimes inadequate on some computer cases, more likely if it's an el-cheapo case, or an older case you installed a newer mboard in.
If the case has additional USB ports up higher that are in a USB hub, or if the lower down front ports are not individually connected to one USB port connection on a header on the mboard, not all devices work when connected to a USB hub that has multiple ports connected to only one USB port connection on a mboard header.
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1: