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I use a 2 TB drive for Time Machine and have used the disk for about a year or two.

Today I wanted to check on an old file (an old version of MonoDevelop) and found out that my oldest backup is from January this year.

I checked the disk itself and found that it really only contains folders for dates starting in January.

What did Time Machine do to my old backups???

(There was always enough room on the disk and there are 1.4 TB free now. I never deleted anything in the backups.backupdb folder.)

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Any special changes you made to your computer setup beginning of January? –  patrix Feb 4 '12 at 14:38
    
No. I moved it from Ireland to Switzerland in November. But the Time Machine disk came with it and I just continued backups. Nothing special about that day in January (13th, ironically) as far as I know. –  Andrew J. Brehm Feb 4 '12 at 15:53
    
I just noticed that that is two days after one of my hard disks (which wasn't backed up) stopped mounting: apple.stackexchange.com/questions/36021/… –  Andrew J. Brehm Feb 4 '12 at 15:54

2 Answers 2

The system will delete old backups if it has an estimate that it will need the space for the impending backup. Normally the system over estimates a backup by 2 to 4 GB in routine use, but I have seen some cases where it had estimates of 250 GB or more due to some folder changes. Even without an error in estimation, you could have attached an external drive that increased this estimate drastically.

Only you have the logs and data to know how plausible this is. Check out BackupLoupe if you like to have better measurements on how each backup differs in size from the others.

For this reason, I periodically duplicate my Time Machine drives (or choose to rotate them monthly) so that if there is a bug, I can look when I swap in the new drive to make sure I don't lose some of my older history.

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I thought that might be the case, but the disk always had 1 TB free... –  Andrew J. Brehm Mar 2 '12 at 8:55
    
The most likely candidate is time machine estimated needing just over a terabyte for a backup. Odd, but less odd than the alternatives - like some other process deleting the backups in reverse time order or just random directory or disk errors taking the same form. –  bmike Mar 2 '12 at 14:18

bmike's answer is correct. Time Machine will delete backups if there isn't enough space on your hard drive. However, he did leave out a key bit of information. There is a checkbox in System Preferences>Time Machine that tells Time Machine to notify you when it deletes old backups.
Notify after old backups are deleted
IIRC, this checkbox is not checked by default, so it probably isn't on.

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Note the phrasing of the option: "Notify after old backups are deleted". There's no way (AFAIK) to get it to tell you before deleting them, let alone giving you the option to tell it not to delete them. –  Gordon Davisson Mar 2 '12 at 6:33
    
I have seen cases where this warning didn't fire even when massive deletes are happening - sadly it's when time machine is taking a long time deleting your backups before starting and people stop that backup for whatever reason. I wish the checkbox said something like: stop and warn before X GB is deleted or before X weeks or before X fraction of the backup drive. Any of these more fine (and user configurable) measures combined with the stop before would make this issue moot. –  bmike Mar 2 '12 at 14:30
    
@GordonDavisson I know what that means, but unfortunately, short of doing some sort of hack, that is the only option that's available. Bmike: I completely agree!!! I don't get why it was made that way, but at least there some sort of warning, even if it is after it was deleted. –  daviesgeek Mar 2 '12 at 20:11
    
@bmike, daviesgeek: Yup, that's high on my wish list for TM features. (Other entries: non-klugy ways to change the backup schedule and rotate between multiple backup destinations, and server-side logging when backing up to a server.) –  Gordon Davisson Mar 2 '12 at 21:47
    
The pragmatic solution: Will Apple change this soon? No. Will I buy a second drive and rotate Time Machine destinations bi-monthly or quarterly if I really care about older backups? Yes. –  bmike Mar 2 '12 at 23:19

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