For USB loader gx in vWii mode, can an external 500gb drive with a y-cable be substituted for a 256gb usb thumb drive? I’m curious about this as it would free up a USB port that can be used for the Wii U side of things.
A thumbdrive (unless it's a REALLY high quality one) will be really slow. If you really need the USB port you can get a powered USB hub.
Also, if you want that USB port for the Gamecube adapter, you can plug only one of the two plugs and it will still work, you just won't have rumble (not sure which one you need to plug, just test both).
Absolutely it can and should be substituted, you're better off with a hdd than a usb stick. You can use the back two ports for vWii and two front ports for Wii U if you want to use a hdd for Wii U you'll need a y-cable as well (unless you get one that plugs into wall) You'll need to use ustealth to make both hard drives work at the same time without that formatting nag you'll get.