Have you tried running sdcard_debugger on your equipment?Ok, now where finally getting somewhere!! I put an ISO in a "dvd" folder on a USB stick and it WORKED!!!
OPL did the spinning circle thing during start up and it recognised it!!
I reformatted the SD using the program that R1vver suggested and checked the partition style (which is MBR) tried it out again but still nothing, nothing detected, so it looks like the problem possibly lies in the MX4SIO (even though the green light and activity light flashes) or it's something to do with the SD card compatibility
I'm using a Kingston 64Gb Canvas Select Plus mini SD inside an adapter which i thought was on the compatible list but i could be wrong
https://www.psx-place.com/threads/testers-needed-mx4sio-wip-sd-compatibility-thread.39479/
Perhaps these will determine whether it is the card or the adapter?

i thought someone said that blank allocation was supposed to be there back in this thread. I'll see what i can do