20Gb Shows As 8Gb in BIOS

I have recently installed an IBM Deskstar 20Gb drive into a friends machine and it shows as only 8Gb in both BIOS and under Partition Magic 5. I suspect that the BIOS of the motherboard may require upgrading as it appears to be quite old. (The slotto feed in coal for steam power kind of gives it away) but the user doesnt want to spend any more money upgrading to a better mobo and CPU. Any other suggestions from my fellow support blokes and blokesses?

I’m still trying to get him to give methe BIOS ID from the bottom of the POST screen but it’s taking him a while!

Regards
R Young