Apple - 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.

If your drive does not show up in Disk Utility then it should not be the 'mounting' issue but more likely some in lower level. You need to plug some other USB device to test if your USB ports actually work or not.

I hate to post a solution like this but I replaced the drive with a new one from the store and it works. It was a hardware problem with the drive.