Doesn't your motherboard have at least 4 sata ports? Why not just hook them up internally. You can then use them individually or if your motherboard supports raid, you can raid them. I have 4 internal drives (non-raid) and two externals and my board is fairly old (gigabyte EP-45 UD3R).
The first issue I see with your statement RONNCC is that you mixed HDD with USB. As Hawkeye22 mentions, your motherboard should have enough ports to connect them inside your computer. The question is do you have enough power to power them off your current power supply.