I have been using this firm's Beyond Compare program for over 10 years to keep my (now) two mirrored 2.5" bus powered external backup drives truly in synch. Today, 09FEB15 I ordered their upgrade from v3 to v4 for $15. The software is lightning fast and very easy to use. It has viewers for an incredible number of file formats which efficiently find the differences between non-matching files, even pictures. Email to Scooter Software has always gotten a rapid and complete response.