Oddly, the only one I'd consider from Costco's Western Digital line-up would be this guy (www.costco.com), which while it can do the right RAID1 mirroring between disks, it's only available with a Thunderbolt cable (Apple's new proprietary cabling standard, currently somewhat unavailable outside of the Apple world).

Additionally, this doesn't appear to be network accessible, meaning that it would only be a higher-reliability external drive. Not a bad thing to have, but having something available for services on a home network is pretty valuable (to my use cases, at least).

Costco looks like it has the "storage routers," which allow you to replace your router with a device that can have an external hard drive connected to it, and serve up files and services from that drive. However, no telling if these support the kind of RAID1 redundancy one would want to pursue.