try y-cable (even if it's usually not needed for flash drives).
"is another story" is not a proper explanation of the problem.
what do you see ? what it does ? any error message ?
based on what really happen, when/where, etc. it's all a different cause and solution.
- is it only a problem with USBGX, or also other wii homebrew ? (wiimc, wiixplorer, wiiflow, etc.)
- does it work, but not always, is unstable, is not seen at all ?
- does it freeze, or skip the init process and load "channel mode" ?
- did you try IOS58 in loader's setting : Loader's IOS : 58 (it usually fix HDD init problem)
- did you plug it in the correct USB Port ? Wii and vWii are REVERSED ! wii is bottom port, WiiU is TOP-BACK port