How does time machine handle deleted files

Question

How does time machine handle deleted files

I've recently grown into a 1.5TB external hard drive where I back up my iMac through Time Machine. I'm a photography hobbyist who does a lot of processing work on my photos in Photoshop, therefore I'm regularly generating sizable data to back up.

My question is, using some hypothetical numbers:

Pretend I create 50GB of new data by importing a lot of photos onto my iMac, then Time Machine does it's thing before I can decide what photos I want to keep or trash. I later go back and edit my photo catalog and decide I only want to keep 1/2 of photos from that previous import, reducing that data to only 25GB. Will Time Machine keep those old pictures on my external drive, even though I deleted them from my iMac? Will Time Machine ever recognize I threw away those files, assume I don't want them, and overwrite them? Or is Time Machine going to keep using my my external drive to back up everything until I used up all of the externals space?

Message was edited by: Michael Streubert

Powerbook G4, Mac OS X (10.4.2)

Posted on Jul 30, 2010 10:15 AM

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Answer 1

Király

Level 6

10,017 points

Jul 30, 2010 12:34 PM in response to Michael Streubert

Understanding how TM deletes things can be difficult, but I'll try to summarize it:

-TM will back up your system every hour.

-After 24 hours, TM will delete all but one of the 24 previous hourly backups. The one it decides to keep will become your "daily" backup. You have no control over which of those 24 backups it will select to keep as the daily.

-After 30 days, TM will start to delete the old daily backups too. It will keep 4 of the last 30 backups, one for each week.

-After your TM disk fills up, it will start to delete old backups to make room for new ones.

So, after you delete them from your main HD, the length of time that they kept on the TM drive can vary. If the files existed for only a few hours, then there is a good chance that they will be purged from the backups the following day. Or they might be kept for weeks or months, depending on if TM decided to keep those backups as the daily or weekly. It's kind of a crap shoot.

If the files existed on your main hard drive for between one and seven days, then they will remain on your TM backup drive for at least a month. After that they are at risk of being purged out.

If the files existed for more than a week, they will stay on your TM drive until it fills up. After that, they are at risk of being deleted.

I hope this helps.

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Answer 2

Jul 30, 2010 2:47 PM in response to Michael Streubert

Thanks, that makes sense.

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Answer 3

Pondini

Level 8

38,863 points

Jul 30, 2010 3:35 PM in response to Király

Király wrote:
. . .
-After 24 hours, TM will delete all but one of the 24 previous hourly backups. The one it decides to keep will become your "daily" backup. You have no control over which of those 24 backups it will select to keep as the daily.

It's the first of the day (ie, after midnight).

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