I have 5000 pictures and other files in a directory taking up 35 GB. I want to duplicate this directory.

Method 1: I do a simple copy and paste of the directory in explorer. I have the habit of checking the checksums after copying important files. In this case I noticed that around 2000 files failed the MD5 test. At a closer inspection of a randomly chosen JPEG with different checksums it turns out that some XMP metadata had changed. In particular, the tag

<MicrosoftPhoto:DateAcquired>

had changed the date from 2009 to today (possibly around the time I was copying the files). I have no idea what triggered this XMP data to be changed and exactly when it was changed and why for these particular files, but at least it seems to explain the checksum discrepancy.

Method 2: As I want the exact files to be duplicated, I tried the program FreeFileSync to mirror the directory, hoping no XMP metadata would mysteriously change. A checksum test in addition to a thorough file comparison test in FreeFileSync lead to two similar but yet different results: 31 files fail the checksum test, 23 files fail the file comparison test. The smaller set is not entirely contained in the bigger set, although many files occur in both. What is alarming here is that not only JPEGs are flagged as altered but also som AVIs, MPGs and a large 7-zip file. Closer inspection of a JPEG indicates that it is indeed corrupt: the bottom half of the picture is simply plain gray. Due to the size of the 7-zip file, I have not been able to pin down the discrepancy.

Note, in both methods, every file has its correct file size after being copied.

Question: Any thoughts on what is possibly going on here? I have never had this problem before, and I am now terrified that files get corrupted after simple actions like copy/paste and file sync. Even if I manage to successfully copy the files somehow, I would still like an explanation to this.