This is going to sound crazy since the cable works when you connect it to the PC, but your symptoms are incredibly familiar to me. What you need is a new USB cable if that's an option with your enclosure. Yes, it was that simple in my case. I have no idea why, but the USB cable that came with my drive worked perfectly fine with PC but just refused to work with the Wii - USB Loaders would detect the drive but then fail to load or wouldn't detect the drive at all until I changed the cable which worked like a charm for whatever reason - must've been interference or something on the cable itself.

If that does not help, I strongly suggest downloading ModMii and running a complete re-mod of your Wii, preferably still on 4.2 because it's easily vulnerable and updating to 4.3 gives no benefits.