Thanks. I just checked my laptop with an OpenSuse USB install and that is recognised and boots OK. So both my computers that can't see Knoppix USB install for booting can boot from USB.

As for age, the laptop is getting on a bit and perhaps there still could be some age issues with it but the motherboard in the other PC is only 1 year old and now has a BIOS that's only 1 month old.

Thanks for the CD boot suggestion. I'm sure that would work but really I'm looking for a persistent (btw, OpenSUSE fails on that for me - works fine just live but my quad core PC seems to grind to a halt with the overlay...) USB drive that would work on most PCs I come across without the need to have a CD and (as things stand and perhaps I'm just unlucky with what I have here to try) a 2/3 failure rate is not too encouraging for that.