For these third-party media, I tried a couple of solutions.
- Internet storage (the best of which, for large amounts of data, seems to be carbonite.)
- Specialised backup software
- Simple folder syncing software by Microsoft (SyncToy)
The specialsed software that I tried were way too complicated or had poor interfaces, so I ditched them.
SyncToy is pretty good. I have six folders on my main hard drive. Each of those folders back-ups to one of two secondary drives. I have scheduled the folders to sync every time I start my PC. In this way, two drives would have to fail or my computer would have to be stolen before I would lose all my data. I can live with that.