use GaryOderNichts vWii nand restorer ... and that doesent remove the cios.
if restoring a NAND dump doesn't remove the cIOS, that's probably because you already installed the cIOS before making a backup.
so, restoring that "cios already installed" backup will not delete them, it'll restore that backup you made at that point in time.
- How is it formatted please ?
this one is usually irrelevant, and forcing users to use fat32 is not right.
only "exfat" is not working, but WBFS,FAT32, NTFS and Ext2, Ext3 and Ext4 are working. not only FAT32.
The same applies for GPT, it works fine. both MBR and GPT are working with both wii and gamecube. unsure about other homebrew though. so, mbr is sometime prefered based on which use the drive is intended for. if it's only wii games, it can go up to 16TB in GPT FAT32, while MBR FAT32 is limited to 2TB. MBR is fine for drive smaller than 2TB.
and as I'm here, I'll also tell "active" flag has no effect on consoles, it's a PC BIOS option only.
At least, be sure users are not using exfat

but usbgx shouldn't even list exfat partitions so there's nothing to launch.
not sure, as the loader could find it, but if no game at all are working maybe using y-cable if it's a HDD.
using the correct USB Port too. (back-top on WiiU)
Using the wrong port either black screen, or reboots. not sure why it's not always doing the same thing.