I also have a Synology DiskStation with 4 drives, which gives you 3 drives of space after accounting for redundancy. I back it up weekly to a WD MyBook 8 TB USB drive.
This is an expensive option, but it has a lot of features, most of which I don't use.
The one that I make the most use of is the Audio Station, which allows my entire audio collection to be streamed to my phone (or sent to a Google Chromecast Audio dongle attached to my stereo). (Their Photo Station and Video Station will do the same for photos and videos...)
The other function that I use all the time is the Download Station, which is essentially a torrent client that runs in the background. Perfect for getting your DIME ratios up!
This is an expensive option and powerful enough to handle the accounts of a small business. Not terribly difficult to set up and run, but it would have been nice to have an expert walk me through it the first time!
If you just need raw storage, a couple of WD MyBook drives would do the trick.