Yosemite, bootcamp and huge disaster

You're screwed. The moment you converted the whole thing to GPT, everything in OS X got nuked.

Keep in mind that you cannot install Windows in UEFI on Macs that are Ivy Bridge or older. You also cannot use the EFI Boot installer on such Macs, as EFI Boot will only install onto GPT disks.

Boot Camp Assistant on these older Macs formats the Windows partition as FAT32 first to be visible to the installer. BCA on these Macs will format FAT32 as hybrid MBR, where it appears as GPT to OS X and MBR to Windows.

Your biggest mistake was that you probably booted into the EFI Boot partition of the bootable Windows installer. The moment you GPT'ed everything, you screwed the entire drive.

Your only way is to boot into Recovery, open Disk Utility, reformat the entire drive as Mac OS Extended Journaled (yes, you'll have to remove Windows first before you can reinstall OS X) and then reinstall OS X.

The mistake started from step 2 onwards, assuming that you created an EFI boot disk.

The reason why UEFI Windows won't work on anything older than Haswell is because these Macs use EFI 1.1, while Haswell and later Macs use a custom implementation of UEFI 2.0 (where all of UEFI 2.0 is incorporated, along with some additional custom stuff).

 

Hold down Cmd+R while booting. That will take you into Internet Recovery.

 

Do you have a second Mac to create a bootable OS X installer?

 

GPT or MBR is for the entire physical hard drive, not the partition. That's why it's broken.

The iMac Mid-2011 will support Internet Recovery if it had all the updates (inc the EFI) installed, otherwise you'll have to go back to the grey recovery disk to repartition the drive, install Snow Leopard then upgrade to Yosemite from there.

Hopefully you've learned from this to be very careful with diskpart! It's the reason that Apple bundle Boot Camp Assistant to setup the drive so you can't bork it using Windows tools that don't understand Mac file systems.

 

First, open Mac App Store on that Mac and download the Yosemite installer.

Then, use DiskMakerX (free app from http://diskmakerx.com) to create a bootable USB stick (use an 8GB stick). The app will walk you through it.

After that, stick the USB into the iMac and then start it up while holding Option/Alt. Boot into the USB stick, open Disk Utility and reformat the entire drive. Then reinstall OS X.

You already lost all your data the moment you converted the partition table into GPT from the Windows stick.

 

Just make sure you keep a Time Machine backup next time.

The next time you do it, use Boot Camp Assistant and install using the optical drive. If you stick to BCA, you will never go wrong.

Some people try to 'outsmart' the system and try installing Windows themselves (i.e. by formatting with Disk Utility instead) and then end up screwing everything instead.