SDF does not support multi users. You need to switch to SQL it is much better and supports more advanced functions.
BTW just curious what makes you say
SQL is definitely not an extreme option in all honesty it is the better option and is quite simple and in my opinion much easier to manage. It just takes a little reading on using it and managing it but its very very simple.
Starting with windows 7 and continuing into windows 8 and beyond the OS protects files in C: more than likely its not able to write to the file on root C:
Here is a quick tutorial on setting up SQL Express 2014 which is the newest version: