Time Machine: how to exclude some files from ONE backup, but not ANOTHER

As mentioned, what you want is not within the scope of how Time Machine works.

Get a different backup utility, such as Carbon Copy Cloner (very well respected; but it is a paid app), or some other backup utility, or even roll your own using rsync or ditto (both are commands shipped with macOS).

You are already using multiple disks, so dedicated one to a different backup utility.

I use Time Machine, Carbon Copy Cloner and SuperDuper! Backup utilities. Time Machine and SuperDuper! to local attached external drives, and Carbon Copy Cloner over the network to my Synology NAS.

Of the 3, Carbon Copy Cloner is the most flexible in selecting what is copied, allowing multiple different tasks, scheduling, and its ability to work either locally or over the network (including securely across the Internet; I actually used to use CCC to backup my Mom’s iMac from 300 miles away to my Synology via an older Mac I used as a backup server).

What you want is not possible with Time Machine. There is only one backup “plan” with one or more backup drives.

You would need a different backup solution to do what you want.

You may want to look at rsnapshot

https://rsnapshot.org

It requires more work to setup, but it generates backups that have a similar layout to Time Machine.

It creates a new backup directory tree for each run, but if the file has not changed since the last run, it just creates a filesystem hardlink to the existing file. So you get the name in the directory, but no extra storage for an identical file.

If the file has changed between runs, it copies the file into the directory.

While rsnapshot can be use to backup remote systems, it can be used on local storage as well.

So each backup has the appearance of being a full backup, each directory tree only has new storage for changed files.

But, but, but you have to do a lot of setup work, and deal with your own scheduling, and work from the terminal. It is a lot of work.

On the other hand, you can tweak it as much as you want with respect to the files you backup.

I have not used rsnapshot in about 10 years. I used to use it to backup the development files on my Linux hosted development system, to my office Mac over the company network (hosted system was in Texas, I was in New England).

BobHarris wrote:

Of the 3, Carbon Copy Cloner is the most flexible in selecting what is copied, allowing multiple different tasks, scheduling, and its ability to work either locally or over the network (including securely across the Internet; I actually used to use CCC to backup my Mom’s iMac from 300 miles away to my Synology via an older Mac I used as a backup server).

on Carbon Copy Cloner 

I have no idea. Trying to make Time Machine different from its design usually results in an unusable backup.