Replacing hard drive on my iMac

OK John here's the deal:

These newer "thin series" systems are a bear to open. Even I have a hard time ;-{ The IFIXIT repairability rating for this system is 3 of of 10.

Review this IFIXIT guide: iMac Intel 21.5" EMC 2805 Hard Drive Replacement. You'll need some special tools here so as not to chip or break the display. After which you'll need to apply a fresh adhesive strips (review the tools listing).

First make sure your system is not setup with a Fusion drive as you will need to take some extra steps to swap out the drive.

As to the HD I would recommend going with a different drive. If you've going this far you might as well improve the performance of your system. How about putting in a SSHD like this drive here: Seagate Laptop SSHD. If you are just trying to match the drive so its the same to sell the system then going with the Toshiba drive makes sense. The price difference between them is not much.

OK, How to backup and prep the new drive.

You'll need to get an external HD to create a Time Machine backup to then restore onto your new drive. Just make sure you have a drive thats big enough.

Before you open the system you'll want to get a SATA to USB adapter so you can connect the new drive to your system externally. If you plan to repurpose the old drive then you might want to get a 2.5" drive case and if you really want to go all out get a Thunderbolt version!

Now once you have the drive connected you can run the Yosemite installer to prep up the drive. Just be careful you don't mess up your internal drive if you want it still to be the older OS.

Once the OS is installed onto the new drive use the startup manager to switch which drive will be booting up your system. Once your sure the new drives is working you can then open the system and swap the drives.

The last step here is to restore from your Time Machine backup and you're done!