[SOLVED] Can I add another hard drive?

Hi there,

I have an oldish Dell that I've been adding stuff to to help it keep up with the competition, but I was wondering if anyone could help me out with something?
In the manual, it says that it supports up to 2 Sata hard drives, (which I have currently). However, there are 4 Sata sockets and there is enough physical space to add another 2 so I was wondering if there was anything stopping me from adding another one?

Cheers,

Mike