I have a server at home running Windows Server 2012R2 with the "Essentials Experience Pack" (or whatever it's called) installed.  Server gets used for client PC backups and media streaming (through PLEX).  Right now the system drive is a small SSD and the media drives are in a storage pool, simple no redundancy, with a 2TB, a 1TB and a 500GB drive (with roughly 2.5TB of media right now).  System drive backs up to an external hard drive and the media storage space doesn't get backed up (as everything on it is also on the HTPC--at least that's the theory).  I'd like to add some sort of redundancy/backup for the media files on the server.

Look into CrashPlan online backup. It is pretty cheap and offers unlimited storage. Might take a while for the initial backup, but subsequent backups would be faster. I prefer to have off-site backups, since they are less vulnerable to certain viruses like cryptovirus, etc.

In this case zoolz backup is what you want.