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Helpful answers
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Mar 26, 2015 5:17 PM in response to boostercbby petermac87,If your Macs came with an OS X later than the version on your USB Bootable installer then it cannot be done without first erasing the drives. Even in that case, you may not be able to go backwards. Your Mavericks installer would definitely not work.What version of Yosemite is on your Installer?
Pete
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Mar 26, 2015 5:18 PM in response to petermac87by boostercb,Not sure what the official Yosemite version was when it shipped, but I've done it to a number of 2014 Airs and Pros where I could at least boot to my drive no problem. Now I can't even get to the setup, it prohibit signs immediately after selecting the drive.
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Mar 26, 2015 5:46 PM in response to boostercbby babowa,It depends on the build of the original OS - Macs cannot boot from a version older than what they came with. Onlly exception are Macs manufactured right around the time of a new OS release, they can usually boot from both. And, if they can, you must erase the drive first.
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Mar 26, 2015 5:47 PM in response to babowaby boostercb,What mode can I boot into to erase the drive then try to boot from USB?
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Mar 26, 2015 6:31 PM in response to boostercbby babowa,First, here is the info on all the builds (which I couldn't get to earlier as I was posting from my iPad):
https://support.apple.com/en-us/HT204319
It appears that 2014 MBA's came with various versions of Mavericks which is why your installer worked there.
As for your current ones, you say they shipped with Yosemite? That would explain why they won't boot from Mavericks. There are two options that I can think of: you use recovery mode (use Command + Option + R to make sure you'll be in internet recovery, which will take a while. Keep keys depressed until you see a spinning globe/progess bar) and erase the drive from there using that Disk Utility (have your Mavericks installer plugged in) - if it's possible you can then install from your stick. If not, you should be able to reinstall within recovery and get the OS originally installed. Second choice: you would need a backup/external drive with the same or later OS version as the MBA to boot from, then use that DU to erase the drive and see if you can install it then.