What is unhealthy terrain data supposed to mean?

MATEK F405 wing in ar900 wing crossfire .
I cannot get this stupid thing to arm . I keep getting “terrain data unhealthy” … which does absolutely nothing to point me towards a solution .
*All the TERRAIN functions are disabled in parameters . SD card is formated FAT32 . I have internet connection but something here is not working !
what is healthy terrain data supposed to look like ? SInce we have nothing to do with loading it , we have no idea how to fix this .
The only way I can fly is too disable all arming checks , and it flies fine .
Firmware is 4.0.9

Is there a detailed way to find out what is going on with terrain data . I know about “Prefetch” to download terrain data . Only problem is …Where is it being stored on the laptop . well hidden files !