re the closure of this question:
This contrived supposed objection, i.e. that the question is not clear, is clearly not the case. If people have a reason to wish to close a question they must give legitimate reasons why. In this particular case, it is clear that I identify a particular requirement the answer to which is not trivial or obvious. The answers constitute a perfectly legitimate answer to a real need which may be useful to many people.
NB specifically for Windows OS
Say the directory in question is D:\My Documents.
On computer A, a desktop, my main machine, I mess around with stuff under D:\My Documents... then someone rings me up and tells me to go somewhere in the next hour, for a week, and to take my laptop with me. My laptop also has a directory D:\My Documents. The vast bulk of the files under there are identical to the files under the desktop's D:\My Documents. Indeed if I can find a good way to mirror, fast enough to make it usable, and reliably enough to make it usable, I can update the laptop each month, in semi-readiness for needing to rush off.
What I have tried:
both machines have Backup and Sync from Google (backing up to different gmail accounts' GDrives). This is not a bad solution, but has 2 disadvantages: 1) B&S can't be guaranteed to back up a particular file within a given time of its being modified; 2) even if B&S has updated all the files, identifying the ones you want (i.e. the ones which differ) and then downloading them would then be no trivial task.
Ease US. If I have a very regular backup plan, say doing an incremental every 10 minutes to an external hard drive, I can hook up that hard drive to the laptop and do a "recovery" operation from the latest incremental image to the laptop D:\My Documents location. Oh dear: intensely slow. It laboriously loops through every file seeing whether it exists (plus I'm not clear whether it does a file comparison on name alone, name + size alone, or name + size + CRC comparison).
SyncBack. This doesn't seem to work either. SyncBack often seems to do things rather confusingly... for example leaving pre-existing files in the destination location which are not part of the mirror's source location when you are trying to do a "hard and destructive" mirror operation to the destination.
Brute force copying: to a USB, for example. D:\My Documents is only about 7 GB in volume currently. It is thus possible just to format a USB stick, copy the desktop's D:\My Documents to it, and rush out the door: the replacement of the laptop's D:\My Documents can thus be done in transit, or on reaching a destination. But this is not elegant. There must be a better way!
Some app which is capable of, presumably, quickly scanning the source location, building a database of the existing directory and file structure, with details allowing a CRC comparison of files, and then quickly updating only those parts of the directory structure which actually need updating, possibly involving a USB drive to do the transfer from the desktop to the laptop.
Incidentally, it's kind of essential that the operation should be able to operate in the other direction too: from the laptop to the desktop, i.e. after one comes home after spending those 7 days away from home.
I have looked around for a solution for some time now... so far nothing really seems to work satisfactorily.
