Need Help : bought a 40GB HDD for my old computer: NOT FOUND

Hi guys.

I bought a used 40GB WD HDD for my old p2 computer that I’m going to transform into an Arcade computer… The thing does not detect it… I plugged it into my computer I use everyday and everything works fine… any ideas ?
Oh, and BTW It used to have a 20GB maxtor hdd which is on last legs which is why I am replacing it.

dual p2 333mhz
4x 128mb ram
ASUS p2L97-DS motherboard