When you plug the USB drives in to computer can you hear them powering up? (You should hear a faint hum as the disk platter rotates.)
Another thing to check is if the disks show up on your Macs in Disk Utility - you can launch this by going to:
Finder (in the Dock) -> Applications -> Utilities -> Disk Utility
You should then be able to see in the bar on the left side of the application if your disks have been detected successfully. If they do not show up here - you can check the System Information application by clicking the Apple icon in the top right, then clicking "About this Mac", then "System Report...". From there, select "USB" and see if the disks show up in the top left panel there as USB devices.
If you could report back with your findings from the above checks I should be able to assist further.
Very sorry yep - top left!
Clearly I hadn't had my morning coffee when I wrote this! :)