I guess you can use it with a HDD 2.5" enclosure for SATA and then you connect it through USB port. (Out of the router box)
Also, if you can fit something like this... or even smaller... (USB 2 SATA/IDE)
BTW, you should use that beast in a notebook, because that HDD is pretty fast. Don't waste its performances in a slow and bottlenecked environment like WL500g router.