A customer reported that Explorer was not showing a folder on their hard drive that they were sure was there.

I asked them to check whether the folder really was there, by going to a command prompt and using the dir /a command.

Turns out the folder really was gone.

The customer went back and retraced their steps and reconstructed what happened.

First, the customer created a folder on their D: called D:\backups\fdrive\spreadsheets.

Next the customer copied two files from their F: drive to the D:\backups\fdrive\spreadsheets folder.

So far so good.

Next, the customer wanted to copy their entire F: drive to the D:\backups\fdrive folder, so they performed the following command:

robocopy /MIR F: D:\backups\fdrive

The customer let this command run for a while, but then the operation started encountering Access denied errors, so they hit Ctrl+C to stop the robocopy command.

At this point, the customer noticed that the spreadsheets folder was gone.

The customer theorized, “I suspect that what happened is that robocopy was matching the directory structure of the F: drive against the directory structure of D:\backups\fdrive, and since my important spreadsheet files weren’t present in the F:\spreadsheets folder on the source, it deleted them from the destination. If I had let the copy run to completion, it presumably would have eventually copied the files from their location on the F: drive to the corresponding subdirectory of D:\backups\fdrive.”

The customer continued, “From a user perspective, it seems that I should have really been alerted by robocopy that the target folder (in this case, D:\backups\fdrive) wasn’t empty, and it should have asked for confirmation that I didn’t really want to lose those files (which I didn’t).”

Well, um, yeah, because that’s what the /MIR option means.

       /E :: copy subdirectories, including Empty ones.
   /PURGE :: delete dest files/dirs that no longer exist in source.
     /MIR :: MIRror a directory tree (equivalent to /E plus /PURGE).

The /MIR option means that the destination folder should be an exact copy of the source folder. The documentation points out that this is the same as /E (copy recursively) combined with /PURGE (remove anything from the destination that is not present in the source).

The /PURGE behavior is by definition destructive.

If that’s not what you want, then don’t pass the /MIR flag.