Just did a wipe of my main drive.

It is a 200GB SATA. After the format it only reports 120-odd GB

No idea why. No partitions. I'm on XP and have both service packs installed.

Any ideas?

http://support.microsoft.com/kb/303013

Windows does that....you can get reg edits to fix it however, did your drive come with a disc?
edit-didn't see funky's post

Yeah, I got to that article before and I have the right stuff but it doesn't tell me how to enable it.

Also I have another drive (IDE) which is 250 GB and is recognised fine.

This all started when an error occured during a W2K installation. It doesn't make sense at all.

The drive is indeed showing up at the limit indicated in those articles, but I can't actually find a way to fix it! This is pissing me off.

Have you looked in the Drive manager at all? ie control panel> administrative tools> Computer management > Disk management .
In there look to see if you can format the rest of the SATA drive.

Well spotted. It was there. Now I just have the task of merging them.

Partition Magic to the rescue.

You don't NEED to use partition magic as long as there is no data on it.

I wanted them merged but disk management had no option to do so. Or rather that I could find.