D1nsm00r

There are a few Linux distros that focus on being used in NAS systems that might be a good option, e.g. OpenMediaVault. Alternatively you could use software like Samba or create an FTP server on the NAS.
 
Prebuilt NAS solutions work in basically the same way, they just tend to be a bit more user friendly with their own UI, cases built for hot-swap, etc.