Mac on Windows 10 blue screen of death
Windows 10 should be installed using a BIOS boot method. Two reasons for this. First, the EFI on iMac12,1 models may not be advanced enough to handle an UEFI boot of Windows. Second, the Windows Support Software contains the drivers for a BIOS booting Windows. You would be better off using your optical (DVD) drive and the Boot Camp Assistant to install Windows. If the Windows 10 installer is in the form of a ISO file, then you will need to burn this ISO file to a DVD. Note: you may need a blank double layer (DL) DVD.

If you which to continue with a flash drive install of Windows, then you will need to follow the accepted answer to the question: How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?

Additional Notes

The  Windows installation media can be stored on a DVD, flash drive or an internal drive. When viewed from the Startup Manager you will see an icon labeled Windows and/or an icon labeled EFI BOOT. Should  should select Windows to BIOS boot the Widows installation media. Choosing EFI Boot obviously EFI boots the media. 

There a two partition tables stored on the internal drive. The first table is the legacy Master Boot Record (MBR) table. Any partition information stored in this table is only used by most BIOS booting operating systems. The second table is the GUID Partition Table (GPT). This table can be used by both BIOS and EFI (UEFI) booting operating systems. (Although, a BIOS booting operating system using the GPT rarely occurs.) When both partition tables are used to store partition information on the same drive the partitioning is referred to a hybrid.

The macOS (OS X) operating system EFI boots and uses the GPT. A BIOS booting Windows uses the MBR table, so the partitioning must be hybrid. An EFI booting Windows uses the GPT, so no partition information can be stored in the MBR table. Actually, in this case the MBR table contains a single partition spanning the entire drive (up to 2 TB) and the MBR table is then referred to protective.

Your Mac need to install a BIOS booting Windows. Therefore the internal drive needs to be hybrid partitioned. If you get a message that the internal drive is GPT partitioned, then you have to either employ the solution to How to convert a drive from the GPT format to the hybrid GPT/MBR format when using High Sierra (macOS 10.13.2) or use the h function from the recovery and transformation menu of the third party gdisk command.

Steps to use gdisk to create hybrid partitioning.


Download gdisk
Disable Systems Integrity Protection (SIP)
Enter the command given below.

sudo gdisk /dev/disk0

Enter the following into gdisk.

r
h
2 3 4
y
af
n
ab
n
07
y
w
y

Enable SIP.


Example output from using gdisk:

GPT fdisk (gdisk) version 1.0.4

Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): r

Recovery/transformation command (? for help): h

WARNING! Hybrid MBRs are flaky and dangerous! If you decide not to use one,
just hit the Enter key at the below prompt and your MBR partition table will
be untouched.

Type from one to three GPT partition numbers, separated by spaces, to be
added to the hybrid MBR, in sequence: 2 3 4
Place EFI GPT (0xEE) partition first in MBR (good for GRUB)? (Y/N): y

Creating entry for GPT partition #2 (MBR partition #2)
Enter an MBR hex code (default AF): af
Set the bootable flag? (Y/N): n

Creating entry for GPT partition #3 (MBR partition #3)
Enter an MBR hex code (default AB): ab
Set the bootable flag? (Y/N): n

Creating entry for GPT partition #4 (MBR partition #4)
Enter an MBR hex code (default 07): 07
Set the bootable flag? (Y/N): y

Recovery/transformation command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/disk0.
Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.