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?

  • 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, 2013 at 22:35
  • I've answered you in the question, since the informations could be useful for another readers Feb 12, 2013 at 23:19

2 Answers 2


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.

  • 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? Feb 13, 2013 at 20:14
  • 1
    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, 2015 at 12:05

Since you do not provide much hard data data it's difficult to speculate what your problem is and that's assuming there actually is a problem, and I am not convinced that there is. You say your first backup is 80 GB, but how much data is actually stored on your drive(s)? Considering TM is "smart" enough to not backup ephemeral data like caches and that you also may have explicitly excluded data from backups such as data that is both stored and managed elsewhere, like things stored on DropBox, I have no idea what 80 GB means. Also, there is a long list of reasons why your second backup is almost as big as the fist. One possibility is that the backup criteria changed since your last backup so that fines which were not included initially now are. The other possibility- already mentioned- is that another sync or backup tool is "touching" your files and making them have dates which indicate the file has been updated when it hasn't. Hopefully, this is not the case because this would be a BAD THING regardless of the circumstances with your backups. Another one is that since you started backing up, maybe you kept a local backup store of an iPhone or iPad. Finally, if you have a large data store (like a database) even a small change to it will update all of the database files (depending on the database) and force an update of it. It will do this every time you update this database.

Honestly, I could go on and on with speculations here because there is so little information. One thing I wonder is how are your update backups working now? Can you show the size of each of the updates? Also, what OS and version are you on? Finally, how big is/are the file system(s) that you are backup up? What are you explicitly excluding?

If you an provide a little more I'll be happy to help.

  • Ty for reaction. 1) This answer is already marked as answered, so it might be better to direct Your efforts to somewhere else 2) If creator of question is not providing some information, there is high probability he doesn't know much about the domain in which his question belongs. You shall provide him with direct, short and easy to understand steps to acquire informations he shall provide to make his question more clear 3) If you don't have answer, but question, do not Answer, but Comment to the original question Nov 6, 2016 at 22:28

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