Why can't I get the Windows installer to launch (Bootcamp)? iMac 13, 1 (late 2012)
Your Mac is suppose to EFI boot 64 bit Windows 10. You should not be using hybrid partitioning when installing Windows 10. You should use the Boot Camp Assistant to download the Windows Support Software. If you copied the Window Support Software correctly to the flash drive, then there should be a $WinPEDriver$ folder in the root folder of the flash drive. You should be able to remove hybrid partitioning  by using the Disk Utility application to Mac OS Extended (Journaled) format the internal partition where you intend to install Windows. Or, you can use the diskutil command to JHFS+ format. You should EFI boot from the flash drive to install Windows 10.

The Problem

You view the Boot Camp partition in the Disk Utility and you see the partition contains either a MS‑DOS (FAT32) or ExFAT volume, as shown below.



When installing Windows, you will encounter the following error message.



This is a indication that the installation drive is hybrid partitioned. You need to remove hybrid partitioning in order to install Windows to UEFI boot.

Solution Method 1

If an action by the Disk Utility or diskutil command results in no Microsoft specific partitions existing on a drive, then High Sierra and newer versions of macOS will remove hybrid partitioning. Since there is only one Microsoft specific partition on this drive, the drive will be erased and formatted Mac OS Extended (Journaled) by the Disk Utility to remove the hybrid partitioning. The result is shown below.



When installing Windows you will encounter the following error message.



A solution is to follow the steps given below.


Highlight the partition where you were going to install Windows, then select the Delete button and the OK button.
Highlight the unallocated space, then select the New button followed by the Apply and OK buttons. The result is shown below.


(Optional)  Since the Boot Camp Assistant has always omitted the MSR (Reserved) partition when I have installed Windows to UEFI boot, I will delete this partition. Feel free to leave the partition, if you wish.


  Note: Microsoft documentation states there should always be an MSR (Reserved) partition.

Highlight the partition where you wish to install Windows, as shown below.


Select the Next button to install Windows. The installer will automatically NTFS format the partition.


Solution Method 2

If an action by the Disk Utility or diskutil command results in no Microsoft specific partitions existing on a drive, then High Sierra and newer versions of macOS will remove hybrid partitioning. Since there is only one Microsoft specific partition on this drive, the drive will be erased and formatted Mac OS Extended (Journaled) by the Disk Utility to remove the hybrid partitioning. The result is shown below.



When installing Windows you encounter the following error message.



A solution is to follow the steps given below.


Press the shift+F10 key combination to open a Command Prompt window. Enter the commands shown below.  If your drive and partition numbers are different, then make the appropriate substitutions. The last exit command will close the window.


Select the Refresh button.
Highlight the partition where you want to instal Windows, as shown below.


Select the Next button to install Windows. The installer will automatically NTFS format the partition.


Solution Method 3

If an action by the Disk Utility or diskutil command results in no Microsoft specific partitions existing on a drive, then High Sierra and newer versions of macOS will remove hybrid partitioning. Since there is only one Microsoft specific partition on this drive, the drive will be erased and formatted Mac OS Extended (Journaled) by the Disk Utility to remove the hybrid partitioning. The result is shown below.



To avoid the problems encountered with the first two methods, the partition will be erased and ExFAT formatted by the Disk Utility application. The result is shown below. With High Sierra and newer versions of macOS, the drive will not be hybrid partitioned by this action.



When installing Windows you encounter the following error message.



The solution is to select the Format button to NTFS format the partition. The result is shown below. 



You can now select the Next button to proceed and install Windows.

A Note About Partitioning

There are two unique partition tables stored on a drive where macOS is installed. The first is a legacy Master Boot Record (MBR) table. This table is ignored by macOS. The second is a GUID Partition Table (GPT). If there are partition entries in the MBR table, then Windows with use this table and ignore the GPT. This is referred to a hybrid partitioning. If a UEFI booted Graphical User Interface (GUI) Windows installer detects partition entries in the MBR partition table on a drive, then the installer will not install Windows on that drive. In your case, hybrid partitioning needs to be removed from the internal drive in order to install a UEFI booting Windows 10.