HD is fine, but Pismo doesn't start

Hi everyone. I'm new to forums so I apologize in advance, in case I do anything wrong.
I have a problem which might look similar to many other postings, but it's not (I guess).

My Pismo used to work well (10.4.2 is the OS). Recently, the Hard Drive apparently died. I get the chimes, screen powers up (doesn't show the apple), hard drive doesn't spin.
Tried restarting from the OS-DVD: I get the apple logo, but no spinning wheel and after 30 seconds the DVD drive stops spinning.

So I read all I can online. reset-nvram and reset-all booting Open Firmware. Hard reset with the button behind the machine. No result.
I open the Powerbook, try re-sitting the processor. Still nothing.
I happen to have 4 HD cables, try them all, nothing happens.

I disconnect the Hard Drive and use it with an external USB HD-Case. Miracle: everything works again. So I think I solved the problem. Wrong.
After disconnecting the HD from the external USB case and reconnecting it internally, same story: disk doesn't spin.
Another attempt: HD placed in the USB case, and another HD (with a valid OS) connected internally. It doesn't work (chimes, screen ok, but nothing happens).
Tried disconnecting the keyboard. No result.

The fact that the Powerbook works normally if the HD is connected externally leads me to believe that:
- the hard drive is ok
- processor and motherboard are also ok
- HD cable is also ok since I successfully used it in a different Powerbook

I'm afraid the HD connector on the motherboard may be damaged, but what do you guys think? Is there anything else I could try? And can a stupid defective HD connector block everything? Am I going to need a replacement for the motherboard? *sigh sob*

Thank you in advance for any contribution.