Reinstalling OSX after previously deleting it and installing JUST Windows 8.1 UEFI

Question

I bought a 13" Mid-2013 MacBook Air just after they came out (in Australia). I then proceeded to wipe OS X completely and install Windows 8.1 on a GUID partition, booting under UEFI.

It took many hours of screaming at the computer in frustration, but eventually it worked.

However, I've just been told by my tutors at uni that the subjects I'm currently studying require OS X unavoidably. Sure, I could just use the uni computers to do my work, but I'd much rather be able to work from home as well, and use my MacBook instead of the 5-year-old iMacs they provide.

So my question is: What would be the most painless way of getting OS X back on to my MacBook?

I'm currently creating a system image of my Windows partition and I was thinking of using OS X Internet Recovery to wipe the HDD completely, create a 15 GB partition for OS X and install it, then recover my system image to a partition taking up the rest of my HDD space. But a few questions spring to mind:

• Will I have to ensure that Windows is the first partition on the disk?
• What about the EFI partition/s needed by (both?) OSes?
• And the system recovery partitions?

I'll keep going with my current plan until it either fails or I get a response from one of you lovely folks advising otherwise.

Answer 1

You should be able to install OS X without disturbing your Windows installation at all. Even though your current setup is horribly unsupported by Apple, their disregard for industry standards might actually work out to your advantage here.

First of all, OS X does not use a bootloader the way Windows and *nix do. The Mac's firmware takes care of that, and it can boot OS X from any device the firmware can see without having to modify any boot code. Apple's requirement that OS X boot from a GPT formatted disk is a purely arbitrary one as it does not need or use the EFI partition at all.

Secondly, even though OS X can't resize or write to NTFS partitions, it does understand what they are and knows not to touch them. If you had a proper BootCamp partition, you could remove and reinstall OS X all day long and it wouldn't bother Windows one bit.

The only unanswered question here is what would a clean install do to your EFI partition? I can't answer that because I've never installed OS X after-the-fact with EFI Windows already on the machine. But in the worst case scenario, I can only see it possibly toasting the EFI partition and that's easy to fix.

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Here's what I would do:

You could just shrink your Windows partition, boot into Internet Recovery, create an HFS partition, and install OS X straight to the primary hard drive. It probably would work fine, but to be on the safe side I would install OS X to a clean external drive first and then clone that back over to your primary drive. That would minimize the odds that OS X will corrupt your Windows install.

Obviously, you should make a backup here. That should go without saying, but there I just said it :-)

1. Get an external USB hard drive or a at least a 16GB flash drive.
2. Boot your Mac to Internet Recovery (CMD+OPTION+R at the startup chime).
3. Use Disk Utility to create a Mac HFS partition on the USB device and install OS X to it. It will be sllloooowww, but it will work.
4. Once the install is complete, boot back into Windows and shrink your NTFS partition. NOTE Windows can do this natively. Do not use tools like GParted or they will mess the drive up.
5. Boot back into Internet Recovery again. Use Disk Utility to create a Mac HFS partition using the free space you just created in step 4.
6. Then, still in the recovery environment, use Disk Utility to clone the partition with OS X on it (not the whole drive) to the partition you just made in step 5.
7. Unplug the USB drive and Option+Boot your Mac into OS X. You should be good to go.

Answer 2

What you describe sounds good. I would do the same, with a couple differences:

Use Internet Recovery to reinstall OS X, but format the entire drive and install OS X over it. Then, use Boot Camp to resize the OS X partition and add one for Windows. In your case, just make the Windows partition rather large (you suggested 15 GB left over for the OS X one). Install Windows with Boot Camp, then recover your files from your backup.

If you do it this way, it's entirely Apple-supported. You don't have to worry about EFI and recovery partitions and the like: OS X will take care of them. Likewise, the position of the Windows partition won't matter; installing Windows via Boot Camp is supported and will work.

The one caveat would be restoring your Windows system. Best results would come from doing a clean install of Windows via Boot Camp, installing Apple's drivers, then restoring your user files and programs. You might need a different backup strategy than a system image.