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Error message stating "Storage is not writable"

This error occurs when you have manually copied a map to an SD card, and the app is trying to add to the map, or repair a corrupt section of the map. If you go to General Settings and turn off "Automatic Map Download", the error message does not appear.  The map will show black squares where the data is corrupted. It will show white squares or a pixelated appearance, where the map is not completely downloaded.
The error message is new in Memory-Map 2.6.5, but the problem is not new. Previously it would either crash or just quietly fail to download.
This is caused by the behavior in Android 4.4 and later. There is a security issue if malicious apps are able to modify images and other files on the SD card. They could place specially crafted code inside a media file, so that if it is opened using a browser, it exploits a vulnerability and gains access to everything the user is doing in the browser (passwords, etc).  So the situation now is that any normal app is NOT able to write to the SD card. Only apps installed by the phone manufacturer (Eg, the pre-installed "Files" app) can write to the card. The user may think the card is writable because they can write to it from the Files app (or from a PC), but that doesn't mean that Memory-map can write to the SD card.

The recommended way to use an SD card is to download a fresh copy of the map from the Digital Map Store, as described here.