windows 98se, 40 gb hard drive and dell optiplex, why not?

Okay, I am so lost I ca’t see the screen anymore.

This comp is old, no doubt,Dell Optiplex GX1 running windows 98se, I flashed the bios already, version A10, latest one I could find.
I connected a second hard drive to the system, I first tried a 160Gb Maxtor, but the computor saw it as a 5Gb drive.
So I loaded a 40 GB drive in, same thing, system reports a 5GB hard drive.

I am replacing my motherboard in my other system after a samall blowout, the Dell is for internet,email,etc.

I just want to add a second hard drive bigger than 5GB.
Whats holding me hostage here?

Thanks for any ideas or advice!
Shane