Question Drive letter not showing up

[endryus]

Hi everyone,
I got a weird problem. On my Win10 PC I need to assign the letter M to one of my hard drives but when I do, it doesn't show up in File Explorer.
I suppose there's some conflict of some sort, because I can use ANY other letter and it will work fine...
I tried to assign the letter M to other drives, both SATA and USB but they disappear from Explorer, even though in Disk Management Utility they show as correctly assigned.
I already used DriveCleanup.exe to remove all entries in the registry at \HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices without luck.
Any ideas?

 

[endryus]

In Disk Management (or softwares like Minitool Partition Wizard, etc.) the letter M is free and I can assign it to that disk or any other disk in the system.
Let's say, the drive is N and I can see it and use it in File explorer... but when I change it to M, it just vanishes...
As I said, it happens with every single drive, so it's not a single HDD issue...