This is what I posted back in October, and I'm still happy with the set up:
I bought an 8 Terrabyte Western Digital "DuoBook" external hard drive set for $350 on Amazon. It has two 4 TB hard drives built in. It seems to work great.
First I configured it to work in RAID configuration, which means both drives mirror each other. That way if one crashes you get a chance to save all your data into another backup from the drive that still works. That means my effective storage is only 4 TB instead of 8 TB.
Second, it has built in software that let me program when to back up my other internal drives. After the initial backups (all done over USB 3.0), the system now automatically backs up any new photos or video every night at 3 a.m.
Third, I can still use it as a normal separate hard drive in Windows Explorer.