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I have a 20gb file that I need to back up and I don't have an external drive or access to any high-capacity cloud storage but I need to format/reinstall OSX Lion on my MacBook Air. On Windows and Linux it's possible to make a partition and just ignore it when formatting/reinstalling but being as ML will get pulled down through the App Store I don't want to risk losing the file. So, my question is: How (if at all) can I make a partition that the OSX installation program won't delete when I format and reinstall OSX?

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You won't lose it regardless even if you keep it on the system partition. Reinstalling OS X (Lion and up) doesn't format the partition. All your settings, files, and content are carried over. –  cksum Jan 29 '13 at 16:44
    
I wouldn't count on an in-place upgrade to not delete files. That may be how it works in the 'best case' but that's not something I would rely on. –  TJ Luoma May 9 '13 at 22:04
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Make second partition on the drive using Disk Utility and as long as you don't install to that partition, it will not get erased by an OS (re)install.

You should be able to live shrink your existing partition and create this new space at the end if the partition view of the disk.

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This is the best option, but please make sure you have a backup first. I have had Disk Utility fail to resize a partition and leave me with an un-usable disk. Only happened once in a lot of times, but once is all it takes. –  TJ Luoma May 9 '13 at 22:05
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