In hindsight I also remembered that OsmAnd~ complained about requiring a storage migration on every start, but otherwise appeared to work fine. In the context of this issue that would make total sense though, since it can still read all the existing map data but creating a test file to check if the storage is writable (I don’t if it actually does that, but it would be the most obvious way to test if the storage is indeed writable or not) would fail – albeit with the wrong error code (ENOENT instead of EROFS).