I post my experience, I think it could be useful for someone....
Installing Windows 10 in iMac 2011: Working video and audio (Through USB installer)
After 3 days visiting several pages with tutorials, forums in which it is mentioned that it is impossible to do it and others in which partially possible but losing partial functionality of graphics and sound totally, as well as searched the online help of Microsoft to consult information about options of some specific commands, I have found the solution.
Apple iMac Mid 2011, without optical disc.
Two SSDs have been installed in each of the available Sata III ports, and an HDD in the Sata II port of the optical unit (optical unit obviously have been removed).
Install the macOS Sierra system in SSD Nº1.
Move the "Users" folder to the HDD (about how to do this there are tutorials on the internet, it is not the subject of this tutorial)
Install Windows 10 on the SSD No. 2, so that the graphics work well, as well as the sound works.
Step 1: Create the Windows 10 USB installation disc
Download the last ISO of the installation of Windows 10 from the official Microsoft page.
Format with the “OSX Disk Utility” a USB with at least 8GB with the following format: MBR / ExtFAT.
Mount the downloaded Windows 10 installer ISO image, and copy all the files to the newly formatted USB (I’ve used “ditto” command to do that).
Step 2: Boot the USB installer.
Restart the computer and keep the “Alt / Option” key pressed.
Choose the USB as the boot unit.
Wait for the Windows 10 installer to load.
Press “SHIFT + F10” to open the command window.
Open the partition editor:
List the system disks:
Select the disk on which we are going to install Windows 10. In my case, this is Disk 1.
Note: Make sure that the selected disk is the correct one.
select disk 1
Now we are going to clean the partition table of the selected disk:
Now create the partition table system, in my case MBR, I do not know if it works as GPT:
Now create the partition in which we will later copy the system boot files (100MB):
create partition primary size=100
Now format it with the NTFS file system:
format fs=ntfs quick label=System
Now assign letter “S” to the partition:
Now mark this partition as active:
Now create the partition in which we will then copy the Windows files. In my case, I want this partition to occupy the rest of the disk, starting from the end of the System partition to the end of the disk:
create partition primary
Now format it with the NTFS file system:
format fs=ntfs quick label=Windows
Now assign a letter to the partition:
Now, before leaving diskpart, observe on the list partition the letter that has been assigned in the partition table the USB disk of installation of windows 10. In my case it is the letter E:
We finished with diskpart, we get out with:
Now we are going to list all the Windows 10 images available:
Note: In my case, the letter E is the USB drive.
dism /get-imageinfo /imagefile:E:\sources\install.wim
Note: Depending on the ISO file you download, you may need to to substitute
Now we are going to dump the first Windows 10 image directly from the USB:
Note: In my case, the letter E is the USB drive and the letter C is the destination partition where the Windows 10 image will be copied.
dism /apply-image /imagefile:E:\sources\install.wim /index:1 /applydir:C:\
Now we will use the bcdboot command, which will enable the system boot. ATTENTION. This point is important since the boot that must be created is BIOS type and not EFI type, since in this last case, the sound will not work.
bcdboot C:\Windows /l es-es /s S: /f BIOS
Note: The option “/l”, in which you specify the language in which the windows installer will start by default, is optional, in my case "es-es". The option “/f BIOS” is mandatory, since the EFI type boot will be installed by default otherwise.
Once all this is done, close the Windows 10 installer that we are using from USB, and restart the computer.
Restart the computer and keep the Alt / Option key pressed.
Choose the Windows option to boot it.
Once here, comes the configuration of windows installation.
Once Windows installed, we can install the bootcamp utilities and drivers, specific for iMac Mid 2011, which can be downloaded from here: Boot Camp Support Software 5.1.5621
Good luck guys!