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 (DS213j) with 2-bays (max 8 Tb storage, i.e. 4 Tb x 2) that uses only 19.8 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 1 Tb Seagate ST1000DM003 drives would add another $140 (for both) and adding 2x 2 Tb Hitachi HDS723020BLA642 drives would add another $320 (for both). Adding 2x 4 Tb Seagate ST4000DM000 drives would add another $335 (for both).
You can then use them in RAID 0/1 fashion and thereby have your own internal backup solution as well. (The DS213J is not hotswappable, though; the DS213 version (about $300) and the DS214 version (about $400) are.
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 8Tb external USB drives), but no RAID capabilities. It supports 32 concurrent users. It is limited compared to the 213 series, though, in the number of integrated servers. (The 213 / 214 series does allow USB external hard-drives, BTW.)
My vote? DS-213J or DS-213.