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Locus Map works with databases. Maps, points, tracks, some settings, backups… all of this information is stored in databases. Work with databases is two-directional - data flow IN and OUT of them. IN = writing, OUT = reading. Furthermore, the reading/writing does not occur in one place of a database - Locus needs to read/write at RANDOM places of its databases.
A typical activity for such random writing is caching or downloading online raster maps for offline use. This is NOT possible on SD card managed by Android 5+ (actually, it began in Android 4.4.2., as you can see >>).
Android 5+ enabled random reading and that is why you can store your vector maps (mapsVector folder), SRTM files (elevation data) and backup files on SD card. ONLY them, not anything else. All other Locus directories must remain in your device's inner memory (Maps, Data/Database, Export, Geocaching etc.)
All you need to know about Locus on SD card in various Android versions is described in detail here >>