I have an issue, which is that when I try to sync an .img file to Dropbox on Android (via DropSync app), the entire file is reuploaded when even a single byte is changed. This doesn't happen with other files. I get the same behavior when using the DB for Windows desktop app. The files were created by Linux Deploy, an app that allows to run a Linux distro on rooted Android devices, in a chroot jail. LD uses the standard 'dd' utility to create the file. The file in question is 6+GB, so reuploading all of it when changes are made is undesirable. Is there something unique/different about img files that causes this? How can upload only the changes?
Dropbox works by splitting the file into pieces (like Bittorrent). I don't know if the iOS or Android apps work the same, but Dropbox will just verify the split 4MB file pieces are identical and not upload it again. The .IMG file is most likely changing the content enough so that the file pieces do not match the previous upload.
Dropbox uses 'delta sync' which means only modified portions of files are downloaded/uploaded, instead of transferring entire files every time they’re changed. However, the Android app DropSync does not support delta sync.
The reason this doesn't work with a TrueCrypt container (or any other encryption program worth it's salt) or some disk imaging solutions is that changing even one bye of the file in the container or disk image is going to change the entire container or disk image. The encryption would be severely compromised if you could change only one byte in the container and only one byte changed in the resulting container file.