Using NTFS hard links to combine full/differential backups
There is a way to make backups like you say, but I don't believe with Robocopy alone.
There are several programs that will hard link or dedupe backups, and be space efficient.
Here are a few programs I've used or ran across:
- Dupemerge - free - While not a "backup program" it is a bulk hard linker.
- hardlink backup - free for personal use - uses hard links
- Duplicati - free - (dedupes I believe like rsync does)
- rdiff-backup - free - dedupes like rsync
I believe what may be a good fit for your questions is Dupemerge. You could have a batch file execute Robocopy to copy the contents to a directory (ex. d:\backups\YYYY-MM-DD). When that command is done, the batch file could call Dupemerge on d:\backups. That would hard link the files within and across all the subdirectories of d:\backups.
The downside of this is you would need enough space on the external to hold the newly created unhard linked backup. After Dupemerge is run, the newly created backup will be hard linked and the space will be freed.
This would make restoring easy because each dated directory would appear to have a full backup.
As a side note, if you are going to use hard links, I like Link Shell Extension. It helps let you know what isn't hard linked, what is and what it is linked to.
I’ve run across other hard link or deduping programs, but I liked these the best.
BackupAssist will let you exactly what you require. Its use of hardlinks is named "Single Instance Store", but it's actually hard links under the hood. It can be used, among other things, to backup to USB drives the way you asked for, or to remote locations using rsync.
We have written our own tool for that kind of work. Its called ntfs-hardlink-backup: https://github.com/International-Nepal-Fellowship/Windows-Tools/tree/master/ntfs-hardlink-backup
For creating the hardlinks its uses the ln tool of Hermann Schinagl: http://schinagl.priv.at/nt/ln/ln.html
So you could also use ln directly. Its pretty powerful