https://www.tomshardware.com/software/windows/bitlocker-reportedly-auto-locks-users-backup-drives-causing-loss-of-3tb-of-valuable-data-windows-automatic-disk-encryption-can-permanently-lock-your-drives
Feeling that their PC was lagging and not working to its fullest potential, u/Toast_Soup (let's call them Soup) decided to reset their PC and install a fresh copy of Windows. It's important to note at this point that Soup had six drives in its system; one of which is the boot drive, of course, and two massive data backups, each worth three terabytes, on the D: and E: disks. After the reinstall, when Soup booted into Windows, the latter two drives were essentially gone, encrypted by BitLocker, despite it never having been turned on in the first place.

See, Soup wasn't aware of the automatic encryption Microsoft enforces these days, so the thought to check first never even crossed their mind. Windows kept asking for a key that Soup never noted down (or even knew existed, for that matter), and without it, the drives are encrypted forever. There's no way to retrieve any data through recovery software — which Soup tried many of — because they can't break the BitLocker encryption.