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I have an OS X 10.6 install copied onto a 16 GB USB key that has a single partition.

I now want to upgrade to 10.8 and keep a copy. I've never used the Mac App Store before, so my first question is: when I buy do I simply get to download the installer?

My main question is: can I easily add the 10.8 installer on my key or does each installer have to have its own partition?

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up vote 2 down vote accepted

It depends on what you want to do.

If you want bootable installer, it has to be on a separate partition. If you want to do that follow "How to create an OS X 10.8 Mountain Lion install drive".

If you don't need a bootable installer, you can just copy the Install Mac OS X Mountain, mentioned at step 2 to any drive you want, including the existing 10.6 install drive, but the result won't be a bootable 10.8 installer. However, you can still install 10.8 by running that file from within 10.6.

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Am I likely to be able to take my existing USB stick with 10.6, and re-partition it without losing 10.6, ideally so the existing partition shrinks to the minimum size needed? – Mr. Boy Sep 19 '12 at 22:01
You should be able to repartition the USB drive without losing data, see this: But as always, have a backup of the data before doing anything fancy with Disk Utility. – lupincho Sep 20 '12 at 4:09

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