If this post seems obtuse to you it’s because it is a note for future development of a real post.
Using the hardlink feature on rsync — on the backup computer one drive takes the main rsync (with -delete) update from the folder/drive to be backed up and a second drive copies that drive with rotating folders using the -hardlink feature of rsync. That way the folder/drive to be backed up is only hit partially for a read, and the real work is done by the backup computer when a whole file needs to be copied because of changes. This is less important for small files but very important for files in the 50 and up megabyte range.
Unison syncs == check out http://www.cis.upenn.edu/~bcpierce/unison/docs.html
Unison might be better than remote mapped drives, especially for large files.
JBackpack is a frontend for rdiff. The problem with rdiff is that getting to view previous versions of a file is tedious. I prefer the idea of just exploring real files, but the drawback to that is multiple copies of large files.