You have probably already tried this, but try all the combinations of jumper settings on the hard drive, master/slave/csel/no jumpers.
Have the computer restart rather than plug when switched on.
Have a look in the disk management tools when the computer is booted, it is not uncommon for a drive to be seen in disk mangement and not be seen in windows explorer.
Also where are you getting the power for the cradle, is it self powered, a powered hub, direct from the usb port.
Just a few things for you have a look at, hope this helps.