I have bought a seagate 120 gb hardrive, i've put it in my pc just underneath my older hardrive. The problem is that the bios doesn't see it. I've set the new hardrive to slave and the old one to master. What could be the problem?

your mainboard is ...
processor ...

try only connecting the new drive and then seeing if it is detected

I have tryed adding the new hard drive by itslef and the pc recognizes it.

My mother board is ASUSTek Computer INC TUSL2-C

you sure you set the jumpers right, b/c its not the hdds

what b/c? i'll try to set the jumpers again.

I think I remember seeing some drives that have a "master - one drive only" and "master - slave present" jumper settings. What is the make and model of your older hard drive?