This HD is having the same problem connected via USB/Firewire, or dircetly via SATA (out of the box):
It spins and it is found by the system, but suddenly (and frequently) it stops spinning (like there is no power) and then restart, until the system cease to find it.
I can only manage to move few MBs from it before it stops, and i have to cut power and give again to have the HD found again.
What could be the cause?
What can i do to fix it, or move "big" files?

Thank you very much.

Firmware or head problems - the second hypotesis more likely so the drive is dying. Have seen many of them lately, and unique solution was for firmware specific tool, for dying head another hw tool to image the drive after setting with the other tool some internal parameters. Had no problems. This continuous spin up/down is very dangerous to a drive in such conditions, so I suggest to stop using it until you can get pro help. It won't cost that much but need a new drive too. P.s. Don't try to do it yourself with a pc, the result will likely be a died drive in your hands...
P.p.s. Before you ask - tool 1 : 10.000$ , tool 2 : about 3.500$ ...