After a reboot of my Windows 10 PC today I was surprised to see this Encrypting File System (EFS) dialog pop up asking me to “Back up your file encryption certificate and key”.
I do have BitLocker enabled for the disks in this computer, but I'm quite sure I haven't used EFS to encrypt any files. It's been a few years since I needed to use Cipher.exe to do anything EFS-related, so I read the TechNet documentation to work out which switches to use to search my drives for encrypted files.
From a CMD prompt it's as simple as running “cipher.exe /u /n”.
Microsoft Windows [Version 10.0.14393] (c) 2016 Microsoft Corporation. All rights reserved. C:\Users\Paul>cipher /u /n Encrypted File(s) on your system: C:\Users\Paul\AppData\Local\AgileBits\OPX4.auth C:\Users\Paul>
As it turns out, the root cause in this case is the OPX4.auth file that is on my system because of 1Password. Since there is no EFS-encrypted files on my PC it was safe to choose “Never back up” for that EFS prompt.