64ST SD Card issues.

I tried to put OSM maps on my SD Card as .IMG files. I had a 64gb card formatted as FAT32 and I have a 32gb card as well. The 32gb card only fills up about halfway 14 gb. But my 64ST wont read most of the maps. it only selects a few maps for me to select in the setup map categories. To choose to disable and enable. It’s not making sense. The card isn’t full The GPS unit should be able to see all the maps installed. Does any one have any info on this issue? I need help.