Formatting the new hard drive

Congrats! These newer thin series iMacs are a bear to open!

OK, there's the deal the drive you took out has the recover partition on it which is not available any more so we need to create a bootable drive of some sort here. The easiest way is to get a USB thumb drive 16 or 32 GB. Using your new 27" iMac download if you haven't already Yosemite from the Apps store. But before you run the installer make a copy of it and put it into a folder on your desktop (just in case). Now run it but be careful! you don't want to reformat your internal HD only the USB thumb drive. Make sure you delete the FAT32 partition and create a GUID disk with a Mac OS Extended (Journaled) formatted partition, and install the OS on to it. Once you get that done restart your iMac and hold down the Option key so get to the Boot manager. Select your USB drive and allow the system to boot up did it work? If so reboot again and klet the internal drive take back the boot up. Now last thing here is to copy over that copy of the Yosemite OS installer onto your USB drive. Now your ready!

OK, setup your 21.5" iMac with the USB drive. You don't need to hold the option key as the only bootable volume is the USB drive. Now we are going to repeat the process this time onto the internal disk. After the install is finished do the same thing we did before to check that it's good and then you're all set.

Let us know if you encounter any problems along the way. Good Luck!