BitLocker is one of the simplest ways to encrypt an external drive on Windows, as long as you're using it with Windows devices. It's just one of many security features in Windows 11.

BitLocker and Device Encryption are critical tools for protecting a PC’s data from thieves. Here’s everything you need to know about the encryption technologies built into Windows 10 and 11.

@Entegy is right. Turn off bitlocker, clone using a proper tool (Clonezilla is my preferred, but Macrium works great). Otherwise, Robocopy won't bring over items like proper partitioning and boot.

Microsoft has confirmed that it had resolved the "65000" BitLocker encryption bug that was affecting almost all Windows 11 and 10 client systems. Interestingly, the fix wasn't deployed recently.