I don't think Wii support SDXC, only SDHC.
does wiisx support partition selection ?
"USB/" looks like it's not allowing you to select any partition, so maybe it's looking at the wrong one.
homebrew are usually numbering partitions when compatible with multiple ones, for example : usb1:/ usb2:/ usb3:/ etc. up to usb8:/
I think there's another PSX emulator, part of the arch suite.
if other homebrew can't find your drive, it's maybe not compatible with the Wii, or might require more power (Y-Cable for dual USB as power source).