method to distinguish main/card storage

Hi, is there a good method to distinguish between the Sony main and card storage filesystems?

At some point I assumed the `Sony_Reader/database/.nomedia` file indicated the main memory, but those files have a different purpose :o.

The "Sony_Reader/database/" dir does appear to have more databases on main, like `evernotes.db` and `images.db`, but I'm not sure this applies to all firmwares (going for gold here ;) ).