How to make my iMac recognize my USB external hard drive?
If your USB hard drive is the bus-powered variety (that is, you don't plug it into the wall for power), then it is possible that it is requiring too much startup current to run on one of your computers.

Some computers are pickier than others about how much current a device can draw, and over what period.

You might try a cable like this:



Basically, you can use power from two USB ports, instead of just one.

If your hard drive has a jack for a power adapter, use that instead.