Edit: Oh, yeah, I forgot about UStealth. good call. but it should ask to format it, unless he deactivated the windows option to prompt format questions. or if he is not on Win8+
there's Ustealth tool on windows to toggle that state, and there's also a homebrew to do it directly on the console.

I guess it's not seing the drive nor partition because if ustealth'd edited partition table, so Badablek is right.
go here https://gbatemp.net/threads/ustealth-wii-u-format-disk-nag-workaround.352786/
you'll find the windows program to toggle the state of your drive's visibility
at the end of the first post there's also "toggle usb", a homebrew you run on Wii to set visibility of your drive.

if you never used Ustealth, maybe what I said will help so I kept it below.


original post I wrote:

I think your drive should be in WBFS partition format, so it's not a known format for windows to list. not a FAT32 or NTFS.
if it's WBFS, there are two possibilities :
- your drive has a partition without any letter set for windows, so it's not listed in "my computer".
- your drive doesn't have a partition at all, it's raw with only WBFS data on it.

in the first case, it's easy to fix.
right click "my computer" icon on your desktop, choose manage > hdd.
there's probably another way to access this tool, but it's the one I know.

in that hdd management, you should see your connected drive with "unkown" as partition, or "raw" maybe.
if there's a partition, right click on it and select "set a letter" choose one and save. now windows will try to mount that partition and will ask you to format it : DON'T !
DO NOT try to fix it if asked, don't make it MBR or GPT, you'll lose your data.
Now just try with WiiBackupManager there should be a letter, and you'll be able to mount that partition and see games from it.

Another idea : wii backup manager has an option to hide some letters, verify you didn't filter any in the settings if windows see the drive letter but the program still doesn't.


if there's no partition, it means the drive is raw with WBFS games on it. it's usually done if you used Linux to manage your drive and put the first game on it.
I thought wiibackupmanager was able to load drive instead of letters, but maybe I'm mistaking. you'll need another tool to manage the drive on computer, one which can mount drives instead of partitions.

edit:
third idea, wbfs2fat should be able to convert WBFS drives without partition tables to MBR+FAT32 without losing any games.
or you can try wit/wwt command line tools, but it's a more advanced way to manage your drive. so, don't worry, there are lot of possibilities to access your games on computer, let's just find what the problem is first.

 

ah, I thought it asked to recreate the MBR.
but I never tried to hide mine so I'm probably wrong.