>We RAR some directories into a single file on different spindle. Those single large files are picked up weekly via USB external (security concerns against on-the-wire-transfer)

Yes, but we need the differential only. This is sort of RoboCopy but over the Internet. So, at first, it is everything. Then, it is only what has changed (removal of items to be synchronized on the foreign location, addition of new items or adjustment of files which have been updated).