Well, folks, a bit of advice.... If you use SyncToy v2.0 or v2.1 be sure to also run some type of CRC-check program to compare source and target regularly. I, and others, have confirmed that SyncToy v2.x does NOT always synchronize files. It seems to be most likely to fail when synchronizinf large batches of new photos (JPG or RAW), but it is NOT limited to that. And it is not as simple as a missing file; the target is created and, occasionally, even has the correct size -- but the CRC counts do not match. Open the photo from the target and the error is quickly noted!
I, and, I believe others, reported this to MS initially during the @.0 beta and again after testing each update. I now use RoboCopy and, in a different use, SecondCopy and for both run automated CRC checks weekly. NEITHER of these has ever failed me. (FWIW, SyncToy v 1.4 is also fine -- and I use that occasionally as well.)