I've tried for hours. Nothing works. Sometimes it says failed to save, others it saves and still will not load. This is what I typed for my final attempt in recalbox.conf.

megadrive.configfile=/recalbox/share/system/.config/retroarch/config/picodrive_libretro.cfg
megadrive.shaders=/recalbox/share/system/.config/retroarch/config/mycrt.glslp

I also tried the unmount / mount commands. Tried saving through menus. Tried Core override. Saving the retroarch custom config says "failed". This is the final step to setting this up, but it seems like all was for nothing. Any help would be appreciated.