Sync files to the cloud, but only upload differential changes? I want to maintain a large file synced between my android device and a PC. I will be doing small changes to this file file all the time on the device, and I want these changes to be reflected on the PC. Ideally the file should not be uploaded in full everytime it changes (because it is so big), but only the differential changes (which are small). Is there a tool  / app that does this? Does Dropbox / Dropsync work like this?

Update: This says that Dropbox only uploads the differential part. But I do not know if the same applies to Dropsync.
Differential sync or Delta sync is a feature that is available only on the PC version and not on Android as explained in Dropbox help pages Delta sync for Android


  Power:....Delta sync means you have to carry out hashing operations on the file, which on pretty much all mobile devices is a non-power optimised operation....
  
  Hashing Overheads for small files: Now, if the file is very large, and the change set is very small, sure there will be a tipping point, but for mobile stuff we aren't talking about a usage pattern which generally covers files hundreds of MB upward in size being edited, its going to be brand new files (photos, videos etc) or small changes to documents etc....You aren't going to be editing huge videos on an Android phone or tablets for example.


(Emphasis Supplied)


If Dropbox for Android doesn't do Delta Sync, Dropsync logically cannot do, since it feeds Dropbox. Besides, app forum talks of comparing files but not differential or Delta sync
I couldn't find any alternative cloud solutions that implement this sync on Android, perhaps for the reason mentioned earlier. Even services like Own Cloud which have an Android app, give similar reasons
Only alternative I can think of is using rsync which does incremental backups, which is faster than differential backup (See a nice tabulated summary here)

Some pointers that may help you implement this solution ( should you decide on this route, it may help to ask a separate question for details )


How it is implemented for USB backup. Sync ext SD to USB OTG **without** using a PC
See footnotes of Izzy's answer for app based solution using rsync for Android and where to download it from
From SE.ubuntu , How to rsync to Android