RAID Arrays for online storage services.
So many services offer a free tier, maybe 5gb for free and then you pay after that. Some are much higher. Build sort of a proxy to these services so that you have a distributed and large free online storage system.
This was an idea my team had last year when we were looking closely at a photo organization and management startup. We had won a startup competition, had investors tender offers but in the end we decided not to pursue the idea primarily because the storage business absolutely sucks, and photo systems are inherently storage businesses. This idea of 'BYOS' (Bring your own storage) was one of the hacks we thought up to get around the problem but in the end customer discovery taught us that the idea had too much friction for most people. Tech folks loved it, 35 year old moms didn't.
You can simply start with a few of the larger players, use the service to connect your free DropBox, Google Drive and OneDrive accounts. There may even be a monetization option wherein as you approach saturation of the storage you push the user to sign up with a specific vendor for a discounted deal and that other vendor can be a partner company or your own storage medium.
It has to be simple and transparent though, you still want people to have that simply sync experience regardless of where the file is stored and they should be able to view all the files across all the services at one time, regardless of where they are physically stored.