Question : Problem: Why would a faulty SATA drive stop my PC doing a POST?

I was trying to add some RAM and an EIDE hard drive to a computer which had been working the previous day, After my attempt to upgrade it, it stopped booting. Not only stopped booting, it did not appear even to get as far as the POST; monitor showing "no input signal" - nothing.
So I put it back to how it was before I started fiddling with it. Same result. No boot, no POST.
Finally narrowed down the fault to the SATA drive. Whenever it is connected to a mobo, the Boot/POST does not happen, when it is disconnected, the mobo works fine.
It is a SATA 150 drive - the older type. The old mobo could only support SATA 150 so it used to work fine on that. The new boards I have tried it on do SATA 300. I don't think that can be part of the problem, but I thought I'd mention it.
So my questions are closely connected:-
What might have caused the symptoms I describe? Is there anything I can do to fix it? Is there anything I can do to copy my data back off this drive, given it appears to cause major issues to any PC it is connected to?

Old PC
3.0GHz Intel P4
ASUS Rock 800MHz FSB mobo
512 MB RAM

New PC
AMD X3
ECS GF8100VM-M5 mobo
3GB RAM

Answer : Problem: Why would a faulty SATA drive stop my PC doing a POST?

Short circuit in the hard drive logic board or a short somewhere in the mechanism?  That would be my guess.

You can try getting another logic board exactly the same as your one.  Must be exactly the same.  Scan eBay for hard disks.

See here how this person did it http://www.deadharddrive.com/

This doesn't guarantee that this process will work for you.