Storing user Database files on their Dropbox or Google Drive

I would like to store the DB files on user's free storage space and access those from my cloud server program (say EC2/rackspace etc.). This is mainly to give affordable service to users.

I searched the forums, however mainly the threads are about backup db files to storage.

I thought about MongoDB & SqlLite as potential db providers for my program. There will be separate db files for users.

SqlLite files can be stored on remote storage space, fetch & load in memory whenever required. This may be slow, as I understand.

Anything similar can be done with MongoDB?

Has anybody doing this (Keeping the live db files on remote storage space) or any thread/post which I can refer.

Any idea/solution to achieve remote storage of live db files?

What could be pitfalls of this?

How can I make sure about security? As its user data, I am ok if user can open the db file, however changing the contents may be problematic. Also if I can encode the db file, it would be great (again slow speed, especially if its MongoDB).

The main intention is to leverage user's storage space (accessible from internet) to give affordable service. The db is required so that I can quickly access the data using its indexing capability (so plain json/csv file may not work here)