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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?

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Are you worried about the estimate on the backup size or actually tracking the data being written? How exactly are you measuring "supposed" and "actual" sizes? In some sense, the entire 80 GB gets backed up each time if you only look at one instant and sum all the sizes. – bmike Feb 12 '13 at 22:35
I've answered you in the question, since the informations could be useful for another readers – Mailo Feb 12 '13 at 23:19
up vote 5 down vote accepted

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.

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What you are saying is not wrong, but can you explain me the difference between tmutil compare result and size of the backup following right after that? – Mailo Feb 13 '13 at 20:14
Thank you so much for this answer. I have exactly this problem and it's Dropbox and GoogleDrive causing the massive (and time consuming) backups. I simply did not know that they touched so many files each time they synced. – Tim Apr 29 '15 at 12:05

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