The mapping app is called Maverick and their instructions are:
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Follow these steps if you want to use external SD card instead of internal for storing maps, waypoints, tracks and other data:
Go to /sdcard/maverick folder
Create folder redirect
For Maverick 2.4+:
Create empty file "to.full.path", for example "to.storage.extSdCard" to keep data in /storage/extSdCard.
Basically Maverick needs to know the name of the external SD card. It will look for the folder called redirect inside folder maverick (usually /sdcard/maverick, should already exists on the internal SD card)
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If I create an empty file called "to.storage.3EB5-F063" in the redirect folder the settings in the app show
Use external SD card tick in box
/storage/3EB5-F063
If there is no file created the Use external SD card and tick box is greyed out.
I must assume I've entered the wrong path name as maps are still stored on the internal storage.
I've moved the app to external storage using Settings, Apps, Maverick, Storage, Change.
Now all the folders except the redirect one are on the SD card.
All Permissions are set allowing read / write to SD card.
Maps that moved over to the SD card cannot be seen by the app.
New folders are generated on the internal storage when maps are downloaded.
I've sent my query to the developers but as yet have had no reply.
It's really starting to bug me now !!
Thanks for taking the time to look