How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?
Installing Windows 10 without DVD or Third Party Tools


  Note: If installing Windows 10 May 2019 Update (1903) Edition, then you should read this question.


This answer applies to Mac models where the following is true.


An optical drive and/or Windows installation DVD is unavailable. Such cases include when the optical drive has been removed or is broken. Also, when a Windows iso file is available and the Mac has a working optical drive, but no blank DVD can be obtained.
Windows 10 needs to be installed to boot using the legacy BIOS method. This generally includes Mac models that where shipped with an optical drive.
Your Mac is capable of 64 bit EFI mode booting from a properly created USB flash drive Windows 10 installer. 


  Note: Unless apple officially supports Windows 10 on your model Mac, there is no documented way of knowing if your Mac can 64 bit EFI boot from a USB flash drive Windows 10 installer. The only way to know is to build such an installer and try booting.



The Windows specifications are given below.


Edition: Windows 10 Pro
Processor: 64 bit
Version: 1709
OS Build: 16299.15


The macOS is version 10.13.2 (High Sierra).

Below, are the basic steps needed to install Windows 10 for an BIOS boot, when the USB flash drive Windows Installer boots in EFI mode.

I have made the following assumptions.


Apple has not supplied the drivers for a 64 bit Windows 10 installation. It is a common misconception that the latest Windows Support Software from Apple should always be used to install Windows 10. This is wrong. Microsoft Windows is designed to use legacy drivers when current drivers are not available or nonexistant. You should use the latest Windows Support Software that apple has released for your model Mac. I assume this is what you get when the latest Boot Camp Assistant application available for your Mac is used to download the Windows Support Software.


  Note: I actually have a 2007 iMac running Windows 10 Pro 64 bit. The software was installed (from a DVD) on a freshly formatted partition using the Windows Support Software designed for a 32 bit Windows 7 installation.

Windows will be installed on the primary internal drive. In other words, the drive with the disk identifier of disk0. 


  Note: Windows can be installed on drives other than disk0, but this may also require a 500 MB "System Reserved" boot partition on disk0. In any case, the procedure to implement this situation is beyond the scope of this answer.

Windows will be installed on the 2nd, 3rd or 4th partition of the drive. 


  Note: Windows can be installed on partitions greater than the fourth partition. Again, the procedure to implement this situation is beyond the scope of this answer.



Below are the installation steps.


  Note: To get a better view of the images shown below, either click on an image or open an image in a new window.



Download the latest Windows Support Software for your Mac. For the 2011 iMac, this software can be found at Boot Camp Support Software 5.1.5621. On my Mac, these files were downloaded to the ~/Downloads/BootCamp5 directory. This software can also be use with the Mac models given below.


MacBook Air (11-inch &amp; 13-inch, Mid 2011)
MacBook Air (11-inch &amp; 13-inch, Mid 2012)
MacBook Pro (15-inch &amp; 17-inch, Mid 2010) 
MacBook Pro (13-inch, &amp; 15-inch, Early 2011)
MacBook Pro (17-inch, Early 2011)
MacBook Pro (13-inch,15-inch &amp; 17-inch  Late 2011) 
MacBook Pro (13-inch &amp; 15-inch, Mid 2012) 
MacBook Pro (Retina, Mid 2012) 
MacBook Pro (Retina, 13-inch, Late 2012) 
MacBook Pro (Retina, 13-inch &amp; 15-inch Early 2013)
Mac Pro (Early 2009) 
Mac Pro (Mid 2010) 
Mac Pro (Mid 2012)
Mac mini (Mid 2011) 
Mac mini (Late 2012)  
iMac (27-inch, Mid 2010) 
iMac (21.5-inch &amp; 27-inch, Mid 2011) 
iMac (21.5-inch, Late 2011) 
iMac (21.5-inch &amp; 27-inch, Late 2012)



  Note: The Boot Camp Assistant can also be used to download the Window Support Software. Look for the "Action" pulldown on the Boot Camp Assistant menu bar. On my Mac, these files were downloaded to the ~/WindowsSupport directory.

Create a ExFAT formatted volume labeled "BOOTCAMP" on the 2nd, 3rd or 4th partition of the primary disk. If this volume already exists, then erase the contents.
Make sure the drive is using the GPT/MBR hybrid partitioning scheme. See my answer to the question "How to convert a drive from the GPT format to the hybrid GPT/MBR format when using High Sierra (macOS 10.13.2)" for a procedure that accomplishes this task.


  User Semi has noted that the Boot Camp Assistant (BCA) will correctly GPT/MBR hybrid partition the drive. So, you may be able to the the BCA to partition the drive, then use the flash drive to install Windows.

Use the Disk Utility application to erase a 16 GB or larger flash drive. Choose the parameters shown in the image below.


Mount the Windows iso file and copy the contents to the "BOOTCAMP" volume. In my case, the label "ESD-ISO" was used to identify the Windows iso. To perform the copy operation, you will need the enter the cp command from a Terminal application window. Below are the commands I usually enter. If necessary, make the appropriate modifications.

cd  /Volumes/BOOTCAMP
cp  -Rv  /Volumes/ESD-ISO/  $PWD



  Note: This command will take a while to complete. Be patient!

Copy the Windows Support Software to the "BOOTCAMP" volume. Below is the command I usually enter. If necessary, make the appropriate modifications.

cp  -Rv  ~/Downloads/BootCamp5/  $PWD

Use the command shown below to rename the AutoUnattend.xml file. 

mv  AutoUnattend.xml  NoAutoUnattend.xml

Copy the contents of the "BOOTCAMP" volume to the "WINSTALL" volume. Below is the command to use.

cp  -Rv  $PWD/  /Volumes/WINSTALL

Disable the ability to EFI boot from the "BOOTCAMP" volume. This can be accomplished by entering the command shown below. This will rename the folder containing the EFI boot files.

mv  efi  noefi

Next, you need to boot from the flash drive. Hold down the option key immediately after restarting your Mac. Release the option key when the Startup Manager window appears. Select the icon labeled "EFI boot". Next, select the arrow below the "EFI boot" label.
Open a Windows Command Prompt window by pressing shift+F10. The result should be similar to the image shown below.


Use the commands diskpart and bootsect to make the "BOOTCAMP" volume BIOS bootable. The commands you need to enter are given below. 


  Note: Sometimes it can take a while for the diskpart command to produce the first prompt. Be patient.


diskpart
list disk


You should see output similar to what is shown below. If there is a * character in the Gpt column for the Disk 0 entry, then you have not successfully converted the internal drive to use the GPT/MBR hybrid partitioning scheme, as described in step 3. If there is a * character in the Gpt column for the Disk 1 entry, then you did not select "Master Boot Record" as the scheme in step 4.

  Disk ###  Status         Size     Free     Dyn  Gpt
  --------  -------------  -------  -------  ---  ---
  Disk 0    Online          500 GB      0 B
  Disk 1    Online           14 GB      0 B


If no * characters appear in the Gpt column, then proceed and enter the command given below.

list volume


From the output of the command list volume, determine the number (###) for the volume with the Label "BOOTCAMP".  Also, determine the drive letter (Ltr) for this volume. In this example, I will assume the number is 1 and the letter is C. Your number and letter could be different. The next command selects this volume. If you determined a different number for the "BOOTCAMP" volume, then make the appropriate substitution.

select  volume  1


The next sequence of commands mark the volume active (bootable) and then quit diskpart.

active
exit


The next command writes the boot code to the MBR and the "BOOTCAMP" volume. If you determined a different drive letter for the "BOOTCAMP" volume, then make the appropriate substitution.

bootsect  /nt60  C:  /mbr

Enter the command shown below to quit the Windows Command Prompt window. 

exit


Press Escape or click the X button to close the Windows installer. If the popup shown below appears, then click on the Yes button. Wait for your Mac to reboot.


You need to finish the installation of Windows 10. Hold down the option key immediately after your Mac turns on. Release the option key when the Startup Manager window appears. Select the icon labeled "Windows". Next, hold down the control key while choosing the circular arrow below the "Windows" label.
Once the installer opens, open a Windows Command Prompt window by pressing shift+F10.
Restart the Windows installation. The commands you need to enter are given below. 


  Note: Sometimes it can take a while for the diskpart command to produce the first prompt. Be patient.


diskpart
list volume


From the output of the command list volume, determine the drive letter (Ltr) for the volume with the Label "WINSTALL". In this example, I will assume the letter is D. Your letter could be different. The next command quits diskpart.

exit


Enter the command below to start the Windows installation. Again, if the drive letter, for the "WINSTALL" volume, is not D, then make the appropriate substitute when entering the command below.

setup  /unattend:D:\NoAutoUnattend.xml

When asked: "Where do you want to install Windows?", proceed as follows. First select the "BOOTCAMP" partition. Next, click on the "Format" button. Finally, click on the "Next" button.
Proceed until you reach the screen where you are prompted for your region. For the 1709 version (OS build 16299.15) of Windows 10, your the screen will appear as shown below.



For other versions of Windows 10, the screen could appear different. As an example, the screen, for the 1507 version (OS build 10240) of Windows 10, would appear as shown below.




  Note: At this point, prohibiting your Mac access to the internet is generally a good idea. For example, unplug any ethernet cables or disable Wi-Fi access that does not require encryption.


Next, press the control+shift+F3 keys to restart Windows 10 in Audit mode. If you are using a wireless keyboard, then you may have to utilize the On-Screen Keyboard. See Ease of Access on the lower left of the screen.


  Note: An unusually long amount of time must pass before the desktop appears.

When desktop shown below appears, the Windows Support Software installer should automatically launch. If this does not occur, use the Windows File Explorer to open the Windows Support Software application named "Setup". This application can be found in the "BootCamp" folder on the "WINSTALL" drive. After the installer application completes, allow the computer to restart.


You will return to the Administrator's desktop. If you are using a wireless keyboard, check to see if the keyboard is paired with the Mac. If not, then add the wireless keyboard to the existing Bluetooth devices.
If you prohibited your Mac access to the internet in an earlier step, you will now want to allow access. In other words, plug in the ethernet cable or connect by Wi-Fi.
Run the Apple Software Update application and install all updates.


  Note: When first executing "Apple Software Update" you may be asked to update "Apple Software Update" itself. If you happens, you should uncheck all other updates and just update "Apple Software Update". Otherwise, you may get an error message saying another update can not be installed. I my case, there were two updates of the "Apple Software Update" to download and install.

(Optional) Download and install Windows updates. You may want to skip this step for now if you believe the updates will be large in number and/or size. However, skipping this step may result in some hardware drivers not being updated.
From the Windows Start menu, select to restart the Mac. You will return to the Administrator's desktop. If the "System Preparation Tool" window is not displayed, then open the sysprep application found in the C:\Windows\System32\Sysprep folder. In this window, select "Shutdown" under the "Shutdown options", as shown below.


  Note: Your screen may also include a window labeled "Boot Camp", which  can be ignored.




Next, select "OK" to shutdown the Mac. At this point, you have completed the installation of Windows 10.

Next time you turn on your Mac, Windows will start in the "Out of the Box Experience" mode. This is basically the way a newly purchased PC starts up, when Windows 10 is already installed.


  Note: When asked, be sure to select an Apple keyboard.



If asked, I can clarify any of the above steps.

Additional Notes


When the window below appeared, I clicked on the Go button.



Next, I check off Enable AMD Video Quality features in supported video player applications, as shown below.



Finally, I clicked on the Apply button, then closed the window.
The Device Manager window is shown below. Here, there appears to be problem with a Bluetooth HID device. I am not sure, but I believe this device is not needed.


  Note: Additional devices (such as a wired mouse and keyboard) are shown that will not appear in the same windows on your Mac. 




This same device appears under on the "Bluetooth &amp; other devices" under "Settings", as shown below. 



I clicked on the device to make the Remove device button appear, as shown below.



I removed the device.
This next problem did not occur when I installed Windows 10 using the above instructions, but has occurred during previous Windows 10 installations. While answering questions, the message below appeared with the error code OOBEIDPS.


  Something went wrong – But you can try again


According to the Microsoft website Windows OOBE fails when you start a new Windows-based computer for the first time, the solution is to do the following.


  For Issue 1 (“Something went wrong – But you can try again”), click Try again at the bottom of the screen. The OOBE process should continue as expected.


Actually, you may have to try more than once.