Yeah, I wouldn't advise using different capacity (or model) drives for RAID5.
Is the IDE drive detected by the BIOS? It's possible you've disabled it, the options for HDs in the BIOS can be quite confusing, at least on my board. I suggest you read your manual to try to figure it out.
Check that you don't need an interim driver during setup, I *think* that one of the IDE controllers on my board needed a 'doze driver before it was recognised. Does your board have one or two IDE controllers?
Jeez my board has a lot of confusing shit on it...
2 different brand IDE controllers
- 1 port
- 2 port
2 different brand SATA controllers
- 1 internal port + 1 external port
- 4 port