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I currently have a MacBook Pro that I'm using at work, but it has now become so slow that I wish to do a reinstall of the OS. It has a recovery partition that offers such a reinstall. I do not want all the content back onto my computer after the reinstall, but I want to keep my time machine backup in case I forgot some important files.

Is there an option so that I some time in the future could be able to get files from an old TM backup?

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Time Machine won't delete old backups until it thinks it needs more space to store a new backup. However, you do not get a warning before the deletion happens - only after the deletion deed is done, does the warning pop up.

You could trust this in the short term, however starting a backup of a cleanly reinstalled Mac is one way to have the system estimate it needs 100% of the side of the drive to start the next backup.

I would not connect that drive to the restored Mac until you can turn off Time Machine and then copy using Finder one of the old backup intervals you wish to save. You could just set the old drive on the shelf or get a new drive to store the "long term" snapshot.

Basically, you don't get notified before the deletion happens, so if you want to keep files, you need to take action now if you are not 100% certain that there is enough space to prevent automatic deletion.

  • So if I just put my TM HDD aside after the reinstall, if I come to think of a file I need, I could get it from my old backup? – user1291510 Feb 20 '15 at 19:57
  • 100% agree. I would go so far as to replace the Time Machine drive for the rebuilt system, so the old stays untouched until you are absolutely certain you have all you need from it. [whilst, of course, still having a backup for the 'new' machine] – Tetsujin Feb 20 '15 at 19:57
  • @user1291510 Yes - and with new terabyte drives costing less than $100 - I usually let my TM drives fill to 90% full and then retire them to the shelf. Once you have several sitting idle, you can copy single backups to one drive and then re-use the other ones... – bmike Feb 20 '15 at 20:01
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    @n1000 safe is not sorry. Replacing the drive is certain to not overwrite it. Backups are cheap, data is irreplaceable – Tetsujin Feb 20 '15 at 20:04
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    @Tetsujin: I fully agree. I would do the same :) – n1000 Feb 20 '15 at 20:05
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If you reinstall and keep using the same Time Machine backup disk, OS X will notice and start a new backup history - i.e. your original backup will remain untouched.

However, in your case I would simply start a new Time Machine (TM) backup:

  1. Archive your current TM (e.g. to a disk image, using Disk Utility)
  2. Test if your Backup is accessible & functional (you can select external TM disks by altclicking the TM symbol in the menubar)
  3. Reinstall and restore data
  4. Delete the current backups / wipe TM disk
  5. Start new backup
  • There are cases where the system will inherit the old backup and not make a new machine directory. I disagree with old backups remaining untouched. Each instance when Time Machine starts - it could decide to pre-thin old backups, deleting files before you get notified that there wasn't enough "space" for the "initial estimate" of how much free space will be required this backup interval. Several clients have spent $$$ for data recovery when they assumed Time Machine won't delete old backups. I very much agree that steps 1 and 2 (archive a backup and test it) are great advice. – bmike Feb 20 '15 at 20:02
  • So far I always had to manually inherit my backups after a reinstall. TM uses the partition's UUID. So maybe if that remains unchanged TM will use the same thread. Not sure. – n1000 Feb 20 '15 at 20:04

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