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I have TimeMachine enabled for my external HDD. I'm triggering backups manually 2 times per week.

I have noticed, that some snapshots, that i made, are gone.

I noticed this behaviour several weeks later, but i had doubts. To make it sure, i've recorded screen video with listing snapshots in TimeMachine.

After watching my screen recording today, I'm sure, that for Augusut i had 9 snapshots. Now, i have only 3 of them.

What's happening ? I had no crash disk reports, errors etc

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Is the external drive getting full? If it's at or near capacity, Time Machine may be making room for newer snapshots by removing the old ones. –  Xanny Oct 12 '10 at 15:18
    
No, my drive has 250 Gb free and with each backup the free space decreasing, as usual. –  AntonAL Oct 12 '10 at 15:26
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1 Answer

up vote 2 down vote accepted

Time Machine will delete snapshots over time. It will retain hourly backups for the past twenty four hours, daily backups for the past month (~30 days) and weekly backups for as far back as it can until the backup disk is full.

Presumably the reason that your snapshots are disappearing is that the August backups have begun to reach the one-month threshold and are now being thinned from daily to weekly.

This is normal, and, to the best of my knowledge, unchangeable behaviour for Time Machine. Time Machine is trying to be an approximation of a continuous backup and thus thins out older backups in order to give more room to allow more recent backups to be more frequent, which helps to sustain the illusion.

If, however, you'd rather have unimpeachable snapshots of your computer, use a different program, such as SuperDuper, rsync, or Disk Utility to make a copy of your hard drive from time to time. That way, it won't be thinned, and you can also do other stuff, like making it bootable.

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I had following backups for August: 18, 12, 11, 10, 8, 7, 5, 4, 2. Now i have, let see ... 25, 18, 11. I think, you're right. I does't knew about this feature. Thanks. –  AntonAL Oct 12 '10 at 15:41
    
OK. When some snapshots are merged into one. What, if i have a daily version of some file ? Will other version be just dropped ? –  AntonAL Oct 12 '10 at 15:56
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Yes. Backups aren't ever merged; they're just dropped - AFAIK the latest eligible complete backup is kept (so, if you have backups at 09:00, 12:00, 13:00, 15:00, 16:00 and 18:30, and Time Machine wants to go to daily backups, it'll drop all but the 18:30 backup). This does have the disadvantage that a file (or version of a file) that only existed from 09:01 to 18:29 will not be kept in the backup at all, so if that file is important to you, make sure you back it up with some other method. As with all backups, there's a trade off between completeness and space taken up. –  Scott Oct 12 '10 at 18:09
    
it's a feature, not a bug! I second the vote for SuperDuper. If you want full frozen backups you'll need to do it another way. –  alimack Nov 3 '10 at 12:17
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