Here are some suggestions for commercially available NAS/RAID devices.
I have been a fan of Synology for years, and they have a NAS (Network Attached Storage) / server (DS212j) with 2-bays (max 6 Tb storage, i.e. 3 Tb x 2) that uses only 17.6 watts for around $200. It can function as many types of file server, and even has built-in media streaming and Torrent, FTP, HTTP, eMule, NZB, and POP3/SMTP/IMAP mail serving functions. It has Directory Server, HiDrive Backup, Mail Station2, phpMyAdmin, VPN Server, Webalizer, Squeezebox Server, Syslog Server, WordPress, and Time Backup. It even supports up to 5 IP cams. It has its own built-in OS (which I believe is Linux-based) and supports 64 concurrent users (and if you've never had the problem of concurrent user crashes with other types of NAS schemes, you don't know what you're missing) and 32 virtual hosts! You may not think you need or want all those things now, but as you progress, you will (believe me).
Adding 2x 500 Gb Hitachi HITACHI HDS721050CLA362 drives would cost another $170 (for both), or 2x 2 Tb Hitachi HDS5C3020ALA632 would add another $260 (for both). (I am amazed how short a time it takes to fill a 1 Tb hard drive).
You can then use them in RAID 0/1 fashion and thereby have your own internal backup solution as well. (The DS212J is not hotswappable, though; the DS212 version (about $300) is).
It has a USB plug, so perhaps you could plug that into your router.
Synology also makes a USB station if you want to only use USB hard drives instead (max 2 x 3Tb external USB drives), but no RAID capabilities. It supports 32 concurrent users. It is limited compared to the 212 series, though, in the number of integrated servers. (The 212 series does allow USB external hard-drives, BTW.)
My vote? DS-212J or DS-212.