Well I'll be.....
Thats even easier than I thought!
I had assumed that the diode went between the circuit and the 12volt rail... so by removing the diode it would just break the circuit.
I guess they run the diode between 12V and ground instead, so if a reverse current goes through it it shorts to ground, killing the diode.
But hey I'm not an electrician.
And yes, removing the diode works.
Thanks for the succinct answer, rameez.
I will probably try replacing the diode once I have got the data off it, as it appears perfectly fine, except for the scorch mark.

I'll probably take a diode from one of the dozen or so faulty WD800 drives that I have lying around, unless someone here can advise on a suitable replacement diode.
Thanks once again, and have a good weekend.
JoshAU