Safedisc

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Bear with me on this one please folks as I'm new to this.
I recently copied 2 games protected with safedisc 2.51 using Clone CD (latest version). They both work fine as straight copies on my machine either using my copier - Plextor 24/10/40A, or my DVD player - Pioneer 116.
I then decide to install them on my son's machine - both install O.K. but will not run, either stating wrong CD rom installed or an error has occurred and windows will have to close. This is using the same copies but playing on his (my old) Yamaha 8X writer or his AOPEN 12X DVD.
O.K. I thought - they are a bit old I'll but him a new writer - but before I do this I'd better check. So, I install my writer to his machine and guess what - same error as he gets - tried my DVD and the same thing.
Both machines are running XP Pro with all updates.
Both have ABIT KG7 motherboards with Athlon XP 1600s.
I have 512 and he has 256 MB RAM but I can't see this making any difference.
Has anyone any ideas please - the only thing I can think of different is the BIOS as his is newer than mine - if it was the other way around I would flash it up but I'm not convinced.
I'll probably swap the BIOS chips tomorrow before I flash his - but can it be this? Or am I missing something obvious?