Yes, Encrypting File System (EFS) can be used to encrypt files on a BitLocker-protected drive. BitLocker helps protect the entire operating system drive against offline attacks, whereas EFS can provide additional user-based file level encryption for security separation between multiple users of the same computer. EFS can also be used in Windows to encrypt files on other drives that aren't encrypted by BitLocker. The root secrets of EFS are stored by default on the operating system drive; therefore, if BitLocker is enabled for the operating system drive, data that is encrypted by EFS on other drives is also indirectly protected by BitLocker.