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.
Seen https://trovebox.com/ ? It started out as a thing kind like this, an Open Source photo service that could use any backend you configured, but now appears to be some kinda B2B service .... shame.
NASUNI http://www.nasuni.com started out with the same idea but now has become much bigger focused on enterprise market. As a Storage Product Manager managing a Storage Virtualization software product over a decade ago, I researched this idea. Also, my first startup was addressing similar problem - consolidation of unused storage space within a geographically dispersed enterprise. Both happened before "cloud" term became popular and S3 wasn't even a service.
Technically, it is a difficult problem to solve if the product need to be software based, simple, transparent and targeted at consumer market so that user can install on their laptops/workstations. The product will need to sit in the middle and capture every operation between OS and DropBox, GDrive, and OneDrive etc. It is not a trivial problem to solve.
A solution that uses a target storage device on a network which in the background sync with different online storage service providers is a better alternative and that is what NASUNI and a few others pursued. But now you are in consumer/SMB storage hardware business. Another better solution option is for users to only install one piece of software (ex: OwnCloud) which uploads to one cloud and then sync with other services in the background. Overall, my impression is that online storage providers offer very little support for such services that try to consolidate storage from different service providers.
On business side, it will be very difficult to make money of people who want to consolidate "free" space from different services. Such people are inherently cheap and wouldn't pay for your service either. So this will lead you to target SMB/enterprise market where typical mindset is to use one service like S3, Box, or Dropbox. So your value proposition has to change from distributed and large online storage system to redundancy, disaster recover, data access and data protection. Such solutions still have the single point of failure - your service. You will need to somehow need to be overcome.
Overall, I believe it is a good idea in principle, figuring out a business model, distribution and customer discovery may be a larger challenge.
If I'm not mistaken OwnCloud provides Dropbox and S3 integration (uploading your files from OwnCloud to these services). Of course this is nowhere close to what you propose, but it appears they take a step into this direction.
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.