Clicking noises from hard drives usually suggest a mechanical failure of some kind. Either because they are underpowered, or because the inner workings have failed.
If no machines are detecting it, then your only recourse would be to take it apart and plug the internal drive directly into a PC, if only to rule out the enclosure as a possible culprit. It would also rule out the possibility of being underpowered as the PC's PSU would be powering the drive.
Of course if the internal drive is actually 2 drives (500GB*2) which is very common then it gets a bit more complex.
Other than that, your best bet would be to take it to a recovery specialist. Though they tend to be very expensive.
----------------------------------
Phil AKA Vacunita
----------------------------------
Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.