Using weekly backup disk to store partial full backup as well?

[Kelly Jones]

I currently backup important files to DVD weekly. These files are 2G
in size total, so I "waste" ~2.7G on each DVD (these are DVD-Rs, so I
can't wipe/re-use them).

How can I use this wasted space to do a complete backup? Example:
first week, backup the "first" 2.7G of my HD; second week, backup the
"second" 2.7G of my HD, and so on. Once the full HD is backed up (over
a number of weeks), roll over and backup the "first" 2.7G again, etc.

My question here: given that the contents of my HD change constantly,
is there a program that will tell me WHICH 2.7G to backup each week?

In other words, a program that will tell me one of the following:

% "Here are 2.7G of files that you've never backed up to weekly DVDs.
Some of these are files that were created/changed in the last
week. Others are old files that you've just never backed up. I'm
choosing these files in a specific way and keeping track of them. Once
you've backed up these files, I'll choose a different 2.7G of files
next week (unless some of the files you backup today change between
now and next week)".

% "There are only 1.0G of files that you've never backed up to weekly
DVD, and here they are. I've chosen an additional 1.7G of files that
you HAVE backed up, but they were backed up a long time ago, so it's a
good idea to back them up again".

Obviously, 2.7G is an arbitrary number here, and it may vary week to
week. If my important files suddenly grow to 3G, I'll have only 1.7G
left for the partial complete backup files.

Improvements would include compression, an exclude list, not backing
up two files w/ the same content, etc.

I do make regular complete backups, but it'd be nice to have this
extra layer of protection.

I considered writing something myself using find/ctime, but am too lazy!

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