If anyone else is running into the same issues, I finally managed to solve it by using a SanDisk Ultra 128GB SD card I had in my camera. Funny that the one regarded as least compatible was the one to work when Kingston, Lexar and Samsung all failed for me.
I've spent the last few days getting setup with FMCB and OPL using MX4SIO SD card reader. It works great with all the 32GB (Kingston) SD cards I've used but I can't seem to find an OPL version that works with both ExFat (rather than Fat32) and my 128GB SD cards. I've tried 3 different SD cards...