So after poking around the documentation for robocopy I figured out a way to accomplish this. It is not bullet-proof, though (see caveats).
What this will accomplish
Computer A has the source directory and computer B has the destination directory. The two computers cannot be connected to each other, but must use a portable storage device to transfer files. This method will allow the source directory to be backed up at the destination directory without copying the entire contents each time a backup is performed. The backup will be an exact copy of the source (see caveats section for exceptions).
For demonstration the following paths are used:
- C:\SOURCE (on computer A)
- D:\DESTINATION (on computer B)
- X:\TEMP (on portable storage device, such as USB hard drive)
Step 1: Create directory structure
We need a way for computer A to be able to figure out what the contents of the backup are, so we use robocopy to create the directory structure on the portable storage device. This will create identical folders and files, except that the size will be 0.
robocopy /e /create D:\DESTINATION X:\TEMP
(note: make sure X:\TEMP is empty or non-existent before doing this.
RC won't create 0-size files if they already exist)
Step 2: Copy new files
Now connect the portable storage to computer A, and use robocopy to copy new files. Note that robocopy treats the 0-sized files as "changed" files if the fully-sized files exist in the source, so we exclude them to avoid copying redundant files that have not changed.
robocopy /mir /xc C:\SOURCE X:\TEMP
(note: the documentation on Microsoft technet says the switch is /xct,
but the copy of RC on my Windows 7 machine says it's /xc)
Step 3: Update the backup
Now that we have the new files copied, we can connect the portable storage back to computer B and use robocopy to finish the job. Note that we use the /XC switch again to avoid overwriting our backup files with 0-sized files.
robocopy /mir /xc X:\TEMP D:\DESTINATION
Caveats
This method relies on the fact that robocopy creates 0-sized files that are otherwise identical to the original files. They are identified as "changed" because they have the same timestamp as the original files but have different file sizes. Under normal circumstances when a file is modified its last modification date will get updated and robocopy will identify them as "newer". If you have files that have their contents modified but not their metadata, the timestamp will stay the same and the result is the above method will NOT backup those files.
Additionally, the portable storage media used can affect the files as well. If you use a FAT32 system, then things like NTFS security info and encryption won't transfer since they aren't supported on FAT32. Also, files larger than 4GB will fail to transfer. So it's best to use NTFS all the way.
