Thanks, Joe, for your reply. I've seen the MacBook only with the 3rd failed disk, a Hitachi 120GB, installed - at that point its keyboard was new and had been tested OK. It would not start up from anything or react to any startup keystroke; the screen stayed white, no Apple or questionmark, refused to eject startup DVD.
First thing I had handy was my own former drive, ready to go with system and all. This worked fine. So did the blank Western Digital HD I put in after and formatted with a fresh System, it passed the Hardware Test.
Later I tried to mount the old Hitachi, connected by an external SATA>USB cable: nothing but clicking noises, neither Disk Utility nor Data Rescue II could see anything: more alive than a doornail but running like a limping headless chicken.
Anyway, not wanting to waste anyone's time: all is well for now, I have to assume it's just an extraordinary case of bad luck.
Best of luck to FineTunedMac!