System and Programs disk.
Defining the system for me: When backing up MY "system disk" it is system AND programs, not just "system".
I want to restore the whole system and the whole programs functionality back.
System Backup: For me backing up the "system" then is not only the base system, and other sub-systems, but also programs, AND parts and pieces of the programs. Many things exist in "USERS" that are "loose data" , and stuff that certannly does not effect the functional recovery of the system,and programs, and its previous ability to operate like it did.
System vrses Loose data: When I use the word "system disk" I am actually referring to the whole operation of the system, and its programs. When I use the words "Data" or Data Disk, I am referring to files and pictures, and documents, and downloads and everything else that does not Break a system.
Loose Data: SO, when moving My own "user" data off of the system (and programs) partition, I prefer it to be only that which does not effect the system and programs, in any way.
all Music files, all video files, all documents, all pictures, downloads for installs. Including "data" that is not a requirement for the system and programs to function, like wallpapers, any large portable programs, or the installation parts (not registry) of games. (just because games not operating correct is not critical to me)
Important User data: Moving Everything in my "user" folder would NOT secure the "system" and all programs, via the one system backup. There are arrays of things in the USER folder that are not Loose Data items, items that are very important to operation of the system and programs. (plus all those settings I made personalising and configuring the system)
The users registry, the programs appdata stuff, the send to, the start menu,the print and net hood, and others.
My junk that could be elsewhere easily: When being intentional about moving "junk that doesn't matter" (to the life of the OS) off the system disk. It is usually the MY junk, that really shouldnt be there to begin with :-). If a person had only One disk, having thier pictures and movies and documents, and loose "user" data on a second partition, makes backing up "the system" simple, and smaller.
Backing up Loose data items Is simple, you can just copy them, they move easily, they dont break anything, they can be stored 20 different ways, in 20 different locations. A few extra copies of this data can be made, securing it in case of a data loss.
Backing up the "system" is not loose data, it has to be where all the pointers point. The data has to corelate with the registry, the shortcuts, the cross links, it has to corelate with the installers and all. Programs that store data all over the system, and in the users location, can break when they do not find all thier parts.
Having many whole incramental images of the system backup can be important to recovering the system fully. If the system disk is of reasonable size and not filled with loose data, it is much easier to have multiple incraments of the whole system for recovery.
When it comes to defining the difference between "the system" and "my users junk" that is how I define it. The important stuff that must be kept perfect or the system and programs fail, and the files junk that doesnt matter where it is.