The 340o doesn't boot - but there is definitely a problem with the HD. It spins up, then down a few times, then there's click and it powers down. Sounds like the heads are stuck, the drive doesn't initialize, and after a few attempts it self parks again. Meanwhile, of course, the question mark icon.
I've had hit-and-miss results from the IDE/SD adapter - it's worked perfectly in the wall street and my PDQs, but not in the Lombard or 1400, so assuming I burn a 3400 install CD and the 3400 CD drive works, it's still not certain the problem is resolved - but it might be, and even if not, we're somewhere nearer.
The 160 doesn't show anything on screen when it's chimed, but the sound from the drive is similar, if rather more muted. I'll try and use a Disk Tools floppy to get it running, but I think there's more wrong than just the drive. However, the 160 was just for parts (I want the bottom casing and keyboard particularly) so I'm not fussed if it doesn't play ball.
And yes, I'll get the 3400 PRAM battery out - thanks for the details! I think I've seen new PRAM batteries for the 3400 for sale on eBay too, so that might be worth a look if I can get it running.