A simple thing to check is that it isn't getting a drive letter that's already taken. Right click on Computer on the desktop or start menu, click manage, then disk management, or storage > disk management if you have vista/7. Here you should be able to see whatever disks the os can see, if your disk isn't shown the problem is something else. If your disk is shown, right click on the partition and then change drive letter and paths, change it to something like M or whatever and see if it now mounts and you can access it in my computer.

Also try in different usb ports. Is the hard drive dock present in device manager when the disk is not in it?