My Passport not recognised

I recently reached capacity on a My Passport 1.0 Tb drive used for storing movies. I bought another My Passport (same model and capacity) and transferred roughly half the movies to the new disk. This worked well until I rebooted the computer. Now it no longer recognises the second disk. I went back to the store where I bought the disk and plugged it into a Mac Book and it was recognised immediately. It also worked on my machine when I got home until I rebooted the computer again.

I have the disks hooked up to a Mac Mini (second latest edition) using the Lion OS. I have tried disk utilities and different cables, USB ports, etc., all to no avail.

Any help would be very much appreciated.

It’s possible that the passport is not receiving enough energy to work correctly, avoid using USB extension cables or USB hubs.