Hey there, I’m also having similar issues.
I have the DMD T-865 and have installed a 128gb SanDisk Xtreme microSD (though I also tried the one that came with the tablet, and another 64gb SanDisk SD card I have).
Running the latest Android Firmware: v12, 5th June 2023
Running latest DMD
1 – Installed micro SD card.
2 – Entered Android Settings > Storage
3 – Selected the SD card > format
4 – Launched DMD2
5 – See a screen where DMD says, “Waiting for SD card to Mount”. This screen stays on indefinitely; after 5 minutes, it was still there.
6 – Hit Abort and switch to Internal Storage.
7 – Go to settings and enable SD card storage.
8 – Go to maps > download > select region (Europe in this case) > select a map > hit Download
9 – App crashes “DMD2 has stopped.”
I’m not sure what else to try, I see no option in the Android storage settings to set it as Internal Memory.