Good news! I tried a different SD 64 GB card made by Kingston and it worked perfectly in MX Linux. I checked the format and it is FAT32. The other 64 GB card made by SanDisk did not work with MX Linux. I will contact them for an explanation and possibly a solution.
If this happens to someone else I hope this helps.
By the way, thanks Michael. I added sudo to mkdir. I was getting a permission warning. If all else fails, try Google instead of driving the moderator crazy