Asked: Can I use 2 USB disk drives at the same time?

Yes. But, Be Careful.

I killed a computer that way.

Yes

USB is designed to let you use 127 devices per controller, and most computers have one controller per port.  Which means that hooking up 2 drives (or more)  isn’t going to be a problem for your operating system.  Most of the time.

But…

You know when you plug in your memory stick and it doesn’t require a power cable?  That’s because USB is a “bus-powered” connection.  Which means that your computer is providing the power to run the thing you plugged in.  This works great for low power devices like a thumb-drive, mouse, keyboard or camera.  But high-power devices, like printers, usually require an external power cord.  And the reason for that is that USB is designed for low power, so the device needs a little help.

Hard drives are in between printers and flash drives.  Spinning the disks requires power.  If the disk comes with external power, you’re free to connect as many of those as you like.  But if the disk uses USB power, there’s a bit of a danger-zone.

My drive drew too much power from the USB port.  Which fried a fuse or something, and killed all four ports on the back of my Mac.  Apple kindly offered to “repair” the dead part for more money than the cost of a new Mac, so I thanked the “Genius” and left.

Be Careful

I could have avoided this with a powered USB hub.  Plug the hub into the wall power, and *if* a device ever misbehaved, this time I’d be down a $10 dollar hub, and not a $700 Mac Mini.  The critical thing is to make sure that it’s got a power source other than the computer.

Sometimes you’ve got your laptop in a hotel room, and you’ve got no choice.  And that’s fine, use the device, it’s meant to work.  But  when you’re home, it pays to be pessimistic.  Most of the equipment you’re using was built by lowest-bid manufacturers, and a little paranoia can save a lot of money in the long run.

• Picture is “Finished 4” by oskay.  Click on the image to see the original