Yeah, don't waste your time, believe me , I also have a Toshiba Canvio 500GB usb 2.0/3.0 and a followed virtually every tutorial out there to make it work , included any tipe of partitioning with the esus partition manager, deactivated sleep mode etc. etc. etc.
I can get it half working, in a sense that like you said, it works with WiiMC and such, but not quite with the loaders.
In my case, when I use CFG or CFG GX I have to unplug it and replug it upon startup, and I found me a nice workaround to that.
Namely, instead of reaching the back of the Wii, and risking damaging the USB port by excessive use, i got myself a USB extension cable, one end to the rear of the Wii where it stays plugged in all the time, one end to the front of the TV, easy to reach and connect/disconnect from the HDD cable, this way, if anything ever gets damaged, that would be the extension alone.
Also you should know that Wiiflow work better with it ( try it ) because it stores all the information in a cache folder and it boots up normally just like WiiMC, listing all the games, every time.
However, when actually launching the games,the result is still random, I got like 50/50 chance of the game booting or failing, but Wiiflow can be set to return to the loader, so I found it more convenient to try to boot the game a couple of times instead of disconnecting the HDD.
So, only workarounds I'm afraid, you should really try to trade it with a different model proved to be working with the Wii, at the end of the day, you save yourself a lot of headache, not to mention multi boot games with different dols ( like Metroid Prime collection ) might still fail to load after the initial game choosing menu.