It could be that BitLocker is on by default--I believe it might be.
I'm not sure why you are getting the message--perhaps something automatic with safe mode? Before proceeding, you can get the BitLocker key through your Microsoft account--try a browser search on "BitLocker key".
I did get the security key last night, but I'm still a bit concerned about trying it.
I was hoping that if I got into safe mode, I could delete those pesky drivers that have been left behind in Windows.old. They are the only things keeping me from getting rid of the Windows.old folder. I get a file in use message when I try to delete them. I've tried Clean System Files, and Unlocker, but they don't work. I was thinking of saving copies of the current files in Windows somewhere else, moving the files from windows.old into Windows, then moving the current files back, but that sounds like it could be a disaster in the making.
You might try a 3rd party utility, such as Gary Utilities, to delete the pesky files. I just checked and it looks like I have some as well, in the Windows.old folder.
Or, just ignore it all, for safety's sake, which is where I've gone.
The directory entries in Windows.old and Windows, for example iaiogpioe.sys, all point to the exact same physical file on disk. That's why they are "in use" whatever you do or name them because the "copy" in C:\Windows is actually used by the system. So renaming and moving the directory entries do not help because they keep pointing to the same disk location.
To clean up Windows.old, make sure the driver files you want to get rid of is in C:Windows\System32\drivers. To clean up just move the file you want to get rid off to the C:\Windows\System32\drivers folder where the running "copy" is. Say "YES" to overwrite, it is the same file anyway, but it gets rid of the extra reference to the file. Repeat for the other files and for the "copies" in the C:Windows.old\WINDOWS\System32\DriveStore\FileRepository.
With the pesky files out of the way you can remove C:\Windows.old completely.