time machine huge backup everytime Sonoma

That's certainly not normal for Time Machine.

You mentioned a large external drive being backed up also, with music files. Not sure if its contents are being changed frequently?

(1) Try removing that external from the Time Machine list of drives to be backed up, just to see if that speeds up Time Machine and stops the backup of TBs every time. If that works, then one solution would be to use SuperDuper or Carbon Copy Cloner to make separate clone type backups for that drive, using Time Machine for the main one.

(1a) Not a bad idea to use both Time Machine and "clone" backup software so you have some diversity in your backup strategy.

(2) Do you have dynamic content stored on cloud services (Dropbox, Google, Microsoft ...etc.)? If so, those get backed up by Time Machine (if they are also stored locally) and if they look "changed" to Time Machine, they will get re-backed up.

(3) Do you have any virtual machines (Windows ...etc)? They can create very large files that are repeatedly backed up by Time Machine anytime you even connect to those systems because the entire file or folder looks "changed."

(4) If you use programs like Adobe Photoshop Lightroom, they often create massive scratch files on the main computer (and a catalog that can become very large) and those can cause very large Time Machine backups.

(5) Try more frequent Time machine backups than every two weeks. Long intervals like that between backups can cause very large and long backup sessions. Not sure why, but this is something I have observed empirically. Daily backups are often tiny, but waiting ~ weeks between backups can mean much larger backups.

(6) If you use MS-Outlook for email, it uses special constructs to store emails and those can lead to very large incremental backups, depending on how your Outlook folders are organized.

I'd like to add one to the list:

(7) If you use any Cloud backup or Could sync on your Mac exclude the folder from the Time Machine backup. For example OneDrive, Dropbox, ProtonDrive etc. might cause you issues (file status updates).

I've seen the above cause 300k updates found and/or 4GB data backed up. This without making any changes to any files in the sync-folder(s) and without running the respective clud-sync software.

Using Terminal command "compare" you should be able to find out what files/folders changed. See: https://discussions.apple.com/thread/254464704?answerId=258371224022&sortBy=best#258371224022

IMHO Time machine is VERY Basic! on every change of OS X or Mac OS Time Machine always makes a new full copy of the new system, at every change of Mac OS to the next! for DAS connected drives I always used Carbon Copy Cloner and network shares I heard good things about Crono Sync.

1

satcomer wrote:

IMHO Time machine is VERY Basic! on every change of OS X or Mac OS Time Machine always makes a new full copy of the new system, at every change of Mac OS to the next! for DAS connected drives I always used Carbon Copy Cloner and network shares I heard good things about Crono Sync.

1

Actually, Time Machine does not make a copy/backup of the operating system anymore. I think one reason has to do with security. It hasn't done that for several generations of the OS. The Apple procedure is to do a fresh install of the OS and then migrate from a Time Machine backup.

Since upgrading to macOS Sonoma Time Machine has shown that it's been backing up about 5.76GB to 1.0 terabytes of data, but Backup Loupe shows that Time Machine has only backed up 2.54GB to 19.01GB of data. IF Time Machine (as shown by its System Preference) was backing as much data as it says it was my backup drive would have already filled up. But Finder says I have 1.71TB remaining with 32 backups residing on my Time Machine backup hard drive. What's going on?

Not true for the newer versions of macOS. TM will backup the user's data volume and any external drives. It does not backup the macOS system volume which is locked.