Sounds like the HDD is fried. Even when it's dead it can often still power up (the motor that spins the discs) but the circuits that read and write are dead. Although i'm no expert on the subject and i have no idea how you can actually make sure. But to me it sounds like it's dead.

The fact that you tried it on both a Mac and a PC and it doesn't even register that a device (working or faulty) is present at all should indicate that it doesn't send or answer signals at all so the PC can even detect that a device is present.