I use a sandisk 32 gig in mine also remember to totally unplug the keyboard from any electric when you install it then once installed format it in in the korg. also make sure to format the card and not the korg disk. It will revert back to the korg disk and you have to reselect sd card before pushing the format button.
Went to Sam's last week, got a package of (2) 64gb Sandisk Ultra cards for $39. Put one in and it works fine; guess I'll use the other on a camera or save it as more backup.
The manual is enormous but all it says is that it uses FAT32 so if the hardware interface is decent then any current card by a reputable manufacturer should be fine. I tend to get SanDisk media for crucial uses because I've never had a failure.