[sciurius]

So I also have to store whether it's relative or absolute.

AFAIK, for all platforms you can reliably determine whether the path is relative or absolute by inspecting the filename. In the Linux/Android case this is whether the filename starts with a slash. I'm sure Windows has similar ways.

If a user stores filenames in the database that are not relative to the MSPro storage location, she creates a non-portable library and must be prepared to face consequences when exchanging the library with other devices.

BTW I don't think "Manage my files" is related to being MSPro storage location relative. I can (and do) instruct MSPro to not manage my files, and still have all of them under the MSPro storage location. Just like BRX described.