Mac – Time Machine is doing very large backups


Background: I bought new drive for Time Machine backup. First backup took about an hour and backed up about 80 GB (let's considered as good behaviour). Second backup was supposed to be above 70 GB (bad behaviour, bad) which could not really be the amount of data actually changed for the while between backups.

When i try manually run tmutil compare in iTerm, it returns a bit more sense making numbers
tmutil compare result summary

"supposed to be above" comes from estimated size in Time Machine preference pane.
"actually 10 MB changed" comes from terminal tmutil compare result.

I think it's wrong it's backing up almost whole drive again. I know the backups consists of actual files and links to not changed files from previous backup. But the links should not consume same size as the linked file and according to free space after second backup the second backup is really consuming the space almost like first(full) backup (checked in disk utility)

Question: How can i prevent Time Machine to make so big backups when actually 10 MB changed?

Best Answer

Time Machine is using differential backups. That means your first backup is a complete backup which is same size with your drive. Starting from the second one backup will include changed files since last backup.

These Backups are accessible by finder. So you can easily find out which folders causes these big backups. After you find the folders you can manually exclude these folders from Time Machine Backup Options.

Some softwares may cause huge backups if they changes files last updated status like Dropbox Agent or similar. You may consider exclude folders of these softwares because they are already copied to the cloud.