it was funny (It was irritating, or sad) seeing everyone trying to find the solution, while I'm saying for two years in all Wii and vWii threads about HDD detection issue to use IOS58 and nobody even mentioned it once !
seems like nobody read other user's problem and solution, and try to help without knowing what they are talking about.
At least, the y-cable was a real help
it's mandatory for WiiU. it shouldn't even be a reason to ask in the forum because all guides tells you to use a yCable. it's as simple as following a softmod guide.
Maybe that's like the NAND and IOS backup, it's there in the guide but everybody think it's useless and a stupid suggestion.
the IOS58 option is not in guides, because it used to work fine with cIOS. it's just a result of new manufactured drives needing that IOS.
vWii softmod is easy :
NO FLASH DRIVE (I guess I'll never stop repeating that because nobody read or try a simple forum's search, or even a simple click and read on existing threads on the first page)
y-Cable, always
IOS58 (which is now default on USBGX beta, because nobody seems to know about that requirement)
done
As for the HDD partition table format : both MBR and GPT are working with a lot of homebrew, but not all.
if you want the best compatibility, go with MBR.
GPT is compatible with USB loaders and nintendont.
USBLoaderGX has a bug with GPT if you have mixed partitions format (fat32+ntfs for example). It works fine with single partition, or multiple same format (fat32+fat32).
one day I'll fix it... or someone else with more free time.
if you want even more compatibility, don't by a drive bigger than 2TB, and preferably old models. New models are using 4096bytes/sectors, it's not working with few homebrew (emuNAND, neek).