Many thanks
Thank you all for these comments.
I was virus-scanning a Seagate Barracuda 160GB drive this afternoon when it began to smoke - now even I know this is not a good sign!
Fortunately, since it was out of the case, I could see the smoke coming from a "BUX C521" and google was then good enough to bring me here.
For the record, the PCB is labelled: 100282774 F 7608UF84
I was reading 1
hm: 12V to Gnd, which translates to a drive drawing 150W or something, if I remember my school physics correctly - evidently the reason for the smoke is revealed!
Not having too many parts to hand, I have just clipped out the offending diode and although this is rather a "penny in the fuse-box" approach, the drive is running fine on a nice smooth power supply. It's backing up now, and as long as it lasts long enough to recover my data, that's fine with me!
I think we can say that anyone with a Seagate drive which won't spin up, and in particular is drawing masses of power should look critically at the diode between 12V and gnd.
Many thanks again.
Chris