I discovered it only by chance, but after spending hours testing and narrowing down the issue, I can easily reproduce it. This strikes me as a very serious issue, and one which would likely go undetected in many cases. The problem is not new, as verification of files copied over a year ago has revealed. I have not found any particular threshold, but I have seen the issue impact numerous files in the 500MB neighborhood. The problem does not appear to impact relatively small files (1 to 100MB or so). Differences in the copied versions were revealed via the behavior of software operating on these files, and are confirmed via WinDiff as well as command-line "FC" (file compare). The USB drives involved pass error checks, and copies made to these drives on other (non W7) systems produce identical copies UPDATE: The issue appears to occur only with NTFS-formatted external USB drives The issue occurs with multiple external USB drives, no matter what make or model. So far, the third party utility "TeraCopy" somehow manages to consistently produce clean copies, and therefore is a temporary workaround The issue impacts copies made via the Windows GUI or via command-line copy or xcopy This would seem to rule out interference by an external program such as an anti-virus program (and the only AV I am running is Microsoft Security Essentials)
Subsequent attempts to copy the same file will ultimately yield an identical copy. The file size of copy is always identical, but content is not No error occurs / no error message appears during the copy I discovered yesterday by chance that when copying a large file (larger than 500MB or so) to an external USB drive, roughly 30% of the time, the copy is not identical to the original.
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