Although the Mid-2011 21.5" iMac is more than ten years old, it has a capable CPU and a nice 21.5" LCD monitor. By replacing the stock HDD with an SSD, the iMac can smoothly run MacOS X and Windows 10.

Unfortunately, Apple no longer provides software updates for this model (MacOS 10.13 High Sierra is the latest supported version). However, Windows 10 can be installed with Boot Camp by using the procedure below. And since Windows 10 will continue to receive software updates from Microsoft until at least October 2025, this iMac can have several more years of useful operation running Windows.

While neither Intel nor Microsoft officially support Windows 10 on a Core i5-2400S CPU, there is nothing in the installation or upgrade process that prevents Windows from being installed or receiving software updates.

• Model Identifier iMac12,1
• 2.5 GHz Core i5-2400S (Sandy Bridge)
  • 4 cores, 4 threads
  • Turbo Boost up to 3.3 GHz
• AMD Radeon 6750M 512MB, 1920x1280
• 12 GB RAM (upgraded from 4 GB standard)
• 4x USB 2.0
• 1x FireWire 800
• 1x DisplayPort/ThunderBolt supports external display up to 2560x1600
• 500 GB SSD (updated from original 500 GB HDD)
• Latest MacOS supported: High Sierra (MacOS X 10.13.x)

I would not recommend installing Windows 10 unless you upgrade the iMac to an SSD.

Replacing the HDD with an SSD immediately makes this iMac a capable machine for general home use.

Before swapping the HDD for the SSD, install the latest MacOS updates on the HDD. This is mainly to ensure that you have the latest firmware installed.

Make sure you get Security Update 2020-006. For whatever reason, this did not go smoothly using Software Update and I ended up having to download and install this update manually.

Once all the updates are installed, view the System Report from About this Mac and confirm that you have the latest firmware:

• Boot ROM: 87.0.0.0.0
• SMC: 1.71f22

DO NOT continue until you have the latest firmware installed as listed above.

Next, create a bootable USB installer to re-install High Sierra after installing the SSD.

Upgrade vendor OWC no longer seems to post a video for replacing the HDD with SSD -- all their videos are for installing SSD in the drive bay behind the optical drive. The drive bay installation is much more involved than the HDD replacement procedure.

I still recommend watching the OWC video as a reference point to see how to do the initial glass panel and LCD removal, along with later portions of the procedure where the LCD and glass panel are put back into place. However, the OWC procedure has you do MUCH more work than is necessary just to replace the HDD.

Instead, use the procedure in this video: Upgrade Apple iMac 2011 21" Hard Drive To Solid State Drive HDD To SSD to see the specifics on how to replace the factory HDD with an SSD.

After replacing the HDD with a new SSD, perform a fresh install of MacOS High Sierra (10.13.x) using the bootable USB installer you created above. It should also be possible to do this over the network, but I did not test network install.

Boot Camp does not allow direct installation of Windows 10 on this Mac. Because of this, the installation procedure is less-than-straightforward, but eventually results in a fully-functioning and quite usable Windows 10 system which can still be booted into MacOS High Sierra if needed. Once Windows 10 is successfully installed, further updates run normally through Windows Software Update. In other words, you only need to run through this procedure once; after that, everything works normally.

Numerous procedures for installing Windows 10 onto the mid-2011 iMac can be found in blogs, message boards, and YouTube. However, I could not get any of those procedures to work on their own. The following is a procedure that finally worked after a lot of trial-and-error. It does not specifically match any single existing procedure that I could find. As with any how-to you find on the Internet, while it worked for me, there is no guarantee that this procedure will work for you.

Also, I am writing this after-the-fact. I kept notes during the process, but I did not take any screen captures. I may not have written down every specific command or step in the process.

After running this procedure, the functionality of the following iMac hardware is fully working in Windows:

• Display
• Audio
• Ethernet
• WiFi
• Camera
• USB
• DisplayPort/Thunderbolt for second display

FireWire (IEEE 1394) was not tested.

First, run Software Update to get all the latest updates available from Apple. Although High Sierra is no longer supported, you can still download a few updates, including Security Update 2020-006. As mentioned above, you may need to download Security Update 2020-006 separately and manually install it if Software Update fails to install the update.

Confirm the firmware versions in About This Mac -> System Report before proceeding:

• Boot ROM: 87.0.0.0.0
• SMC: 1.71f22

DO NOT continue until you have the latest firmware installed as listed above.

• USB flash drive with at least 8GB capacity
• Windows 8/8.1 install DVD
  • The DVD that I created from a Windows 8.1 ISO downloaded from Microsoft refused to work. Boot Camp insisted that it was a Windows 10 DVD. I'm not sure if my ISO was somehow corrupted, or if it contained some newer software that Boot Camp didn't recognize.
  • I eventually found an original Windows 8.1 DVD from a previous Windows purchase from several years ago. Boot Camp finally worked when using this old 8.1 install DVD.
• Valid Windows License Key
  • This is required during the Windows 8 installation process.
• Printed out copy of the procedure below.

I recommend that you read the procedure in full before starting.

• Plug in the USB flash drive
• Use Drive Setup to format it as MSDOS (FAT)
• Insert the Windows 8/8.1 Install DVD
• Run Boot Camp
  • Check both boxes
    • Boot Camp downloads software and copies it to the flash drive
  • Boot Camp asks to partition the drive to allocate space to Windows -- configure as you see fit
    • In my case, I allocated 300 GB of my 500 GB SSD to Windows. If you never intend to use the MacOS partition for anything but Boot Camp, you could allocate even more to WIndows and less to MacOS.
    • Boot Camp reboots.
• As soon as you hear the reboot chime, hold down the OPTION key until you get the boot disk selection screen.
  • Choose the Windows DVD (not EFI) on the boot disk select screen.
• The iMac boots to the Windows installer
  • At the Install screen, choose Custom
  • At the partition screen, select the "BOOTCAMP" partition and click format.
    • DO NOT CHANGE any other partition. The only partition you should touch is BOOTCAMP.
  • Windows Setup copies the installation files to the hard drive and reboots
• At reboot, hold down the OPTION key immediately after hearing the chime
  • Select the Windows HDD this time.
    • Do not select Windows DVD.
    • Since the DVD is still in the drive, Windows will ask if you want to boot from DVD.
      • DO NOT boot from DVD.
    • After a few seconds, it will time out and then boot from the Windows drive.
  • Windows installer continues and reboots again.
• Hold down the OPTION key immediately after hearing the chime
  • Select the Windows HDD.
    • Do not select Windows DVD.
    • Since the DVD is still in the drive, Windows will ask if you want to boot from DVD.
      • DO NOT boot from DVD.
    • After a few seconds, it will time out and then boot from the Windows drive.
    • Windows eventually completes the installation and then goes through several screens before logging into your Desktop.
• Eject the Windows installer DVD.
• Manually install Apple's "Windows Support Software":
  • Go into the USB Flash drive->BootCamp folder and run Setup.
    • Install all the files
• I think you need to reboot after installing Apple's Windows Support Software.
  • As before, hold down the Option key and select Windows HDD.
• Once the Windows Support Software is installed, you should be able to right-click the Boot Camp icon in the Taskbar, and select Windows as your startup disk so you don't need to hold down the Option key and manually select Windows at each boot.
• You can now eject the USB flash drive.
• Run Apple Software Update (from within Windows)
  • This will probably update a couple Apple/Boot Camp files
• You should now have a working Windows 8.x machine
• I strongly recommend that you run software update and get all the latest Windows 8.1 updates (this was several GB of downloads in my case). I am not sure that this is strictly necessary, but it may be useful to make sure the Windows 10 installer does not get stuck on some unexpected configuration.
  • This may take several passes to get all the updates (install updates, reboot, re-run software update).
  • There was one update called "HP USB" that refused to install. Uncheck that specific update and continue with the others.

• While booted into Windows 8, download the Windows 10 Media Creation Tool from Microsoft
• Select the "Create Windows 10 installation media" download
• Run the tool. At the "What do you want to do?" screen, select "Create Installation Media"
  • Do not choose "Upgrade this PC now." Upgrading in place continually failed, apparently due to issues downloading updates during the installation process (possibly because Windows 8.1 is no longer supported by Microsoft).
  • A Windows 10 ISO will be downloaded.
• Double-click the ISO to mount a virtual disk.
• Double-click Setup from the virtual install disk.
• In Setup, there will be a button to "Change how Windows Setup downloads updates"
  • This is very important: Set this to "Not Right Now"
• The Setup process goes through several reboots (the specifics below may not be the same on your machine):
  • First Reboot: "Working on updates:", reboots at 30%
  • Second Reboot: "Working on updates:", reboots at 75%
  • Third Reboot: "Working on updates:", eventually gets to login screen without reboot
  • Once Windows 10 was installed, the display was "stretched" for a few moments until Windows downloaded and installed the proper graphics drivers. Once they were installed (this was done automatically), the screen reverted to its correct settings.
• Run Apple Software Update (from within Windows)
  • There are a couple updates needed for Windows 10
• Run Windows Software Update to get fully up-to-date
• You now have a fully-functional Windows 10 machine
  • You can switch back to MacOS at any time using Boot Camp

Open a GitHub issue if you encounter any problems or have improvement ideas.

• Apple spec page
• EveryMac spec page
• Core i3-2400S spec page
• 500 GB SSD upgrade kit
• SSD installation video

The software and other files in this repository are released under what is commonly called the MIT License. See the file LICENSE.txt in this repository.

1.0 - 2/19/2022 - Initial version based on an actual installation performed in January 2022.
Minor clarifying updates have been made since then, but the overall procedure is the same.