I tried using xcopy and robocopy, but abandoned them when I discovered that they did not faithfully mirror the source to the target.
The aggregate target numbers and sizes regularly didn’t match up, and I am not referring to files that had names or paths that were too long (which no utility can handle), but to GBs of normal files of varying sizes.
I eventually went for SyncBack Free, which I verified mirrored properly.
Like I did with Outlook files, one could use a hard link to put the actual files on a data drive that is completely backed up.