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I'm using robocopy to copy a large number of files (300,000 files totalling around 4TB) from one NAS drive to another. Here is a sample command:

robocopy \\nas1\myfolder \\nas2\myfolder /E /R:1 /W:5

I've done this hundreds of times before across a wide variety of folders and drives without any issue. In each case, as it finishes copying a file, it always reported 100% before moving on to the next.

However, I'm suddenly getting less-than-100% transfers being reported. Usually 98-99% but sometimes as low as 30%. See image:

robocopy file transfers

The robocopy command is still running (expected to take several hours) so I cannot yet determine the validity of the copied files or do any checks.

In what circumstances would robocopy do this? Is there a problem with the transfer or is it reporting partial progress in threads or something?

Update: Small files (< 1k) tend to be worst affected, reporting < 50% in many cases. Large files (> 10MB) tend to report 98%+

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    Windows network shares work with multiple clients. Therefore even if robocopy is running you can check one original and one copied file if they are equal. Just generate the SHA-1 or SHA-256 hash of both files and compare them or use fc.exe /B on command-line for a direct comparison. – Robert Apr 30 at 9:51
  • @Robert thanks for this - I have spot checked several files and they are identical. However, it's hard to verify large numbers of files easily. I'm very puzzled why it is reporting < 100%. I'll post the summary at the end of the command, once it's finished running. – SimonGoldstone Apr 30 at 10:24
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    Verifying a large number of files is easy using sha1sum utility. use it on a directory to print a list of files and their sha-1 hash. Save that output and use it for verifying the files of a second directory. For win32 e.g. available here: lists.gnupg.org/pipermail/gnupg-announce/2004q4/000184.html – Robert Apr 30 at 10:32
  • Thanks, will check it out. – SimonGoldstone Apr 30 at 10:53

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