Hey there, icecappacino!
@jsmithepa is totally right! You'd need to re-format your WD My Passport with a file system that would be recognizable by your router. (
http://products.wdc.com/support/kb.ashx?id=uOlnMy ) You should also check what kind of USB ports you've got on it, or as already mentioned read the router manual or refer to the manufacturer's official website. Either way it's not really recommended to use the external as a network drive. We've got a Wi-Fi mobile storage external drive that is My Passport Wireless. This model wouldn't have any issues being mapped on your router, you can check more about it here:
http://products.wdc.com/support/kb.ashx?id=0oEm28
How to map a WD NAS product on PC:
http://products.wdc.com/support/kb.ashx?id=G0uawn
However, if you decide to use the WD My Passport you've already got with the router, I'd recommend you to keep backups of the data stored on it somewhere else as well, and also keep an eye on its performance and health using Data LifeGuard Diagnostics:
http://products.wdc.com/support/kb.ashx?id=O6ZvyC
Hope I was helpful! Good luck!
SuperSoph_WD