if Windows doesn't let you alter the MBR (probably your antivirus protecting it), you can do it directly on the console.
there's a Wii homebrew which can toggle that hidden state :
USB Toggle.
DarkDesertFox : even nintendo said it's not recommended to use flash drives on WiiU.
there are lot of access, and the flash is not supporting it for long and repetitive access on the same sectors.
that's for the life time part.
additionally to that life time part, USBLoaders and cIOS are not compatible with flash drives in general. either it works in read mode only (no write mode), either it's not even detected.
you are lucky if you find a read mode only device. most users have issues trying to use flashdrives and get themselves blocked into a headache of different things to try to fix something unfixable, while the solution is very simple (HDD), but a lot of users refuse to get an HDD and still persist in trying to use flash drives and are not happy with the result.
HDD are not only for WiiU. you don't like your WiiU that much, it's a thing. But your HDD is not dedicated to WiiU, it's a FAT32 or NTFS which can be used for anything else too. you can store your computer's files on it. it'll always be useful even after you stop using your console.
Any HDD brand should be fine, but if you get one bigger than 2TB I suggest getting a Western Digital drive. it has additional tools which are required for some homebrew. some HDD won't let you format the drive bigger that 2TB using MBR, while it's possible with WD drives.
if you get another drive bigger than 2TB, you'll be stuck with GPT format only, or MBR but limited to 2TB (waste of space).