How to boot into Windows on SSD from macOS
Generally, it is not recommended to boot a Windows installation from a different computer. However, do so has been known to work.


  You should make sure Secure Boot is set to detect and boot from external drives.


You should first determine if the external drive is using a GUID partition table (GPT) or a MBR partition table. This can be done by looking at the output from the command diskutil list. If you see GUID_partition_scheme for the drive, then the drive is using a GPT. If you see FDisk_partition_scheme, the the drive is using a MBR partition table. 

If the drive is using a GPT, then the Mac Startup Manager should detected the boot files on the external drive. Detection of the boot files does not guarantee the Mac will be able to boot Windows. For example, there may be drivers that need to be added to the drivers store. This can be done using the Windows 10 installation media.

Your model Mac can not boot from Windows installed on disk that is using a MBR partition table. If the drive is using MBR partitioning, then there is not enough information posted in your question to tell if the partitioning can be converted to use a GPT. If not, then you might be able to use a third party partitioning tool to make room for adding a GPT. After converting to use a GPT, the UEFI boot file files will need to be added to the drive. Also, you probably should add the drivers from the Window Support Software. This can be done using the Windows 10 installation media.

Update 1: Create an Empty EFI Partition


  The EFI partition is where the Windows boot files are stored.


You can going to need a EFI partition. You can use disk2s1 for this. Below are the steps to backup this volume.

cd ~/Desktop
diskutil unmountDisk disk2
sudo dd if=/dev/disk2s1 of=backup2s1.bin


Next, reformat the partition.

sudo diskutil FAT32 EFI disk2s1


Finally, use gdisk to change the partition type of disk2s1 from 0700 to EF00

Update 2: Create a USB bootable Windows 10 flash drive installer.


  You will need to boot from the flash drive in order to run the commands to create the boot files and transfer the drivers to the driver store.



Use the Disk Utility to erase a USB flash drive. Use the name WINSTALL, format ExFAT and scheme Master Boot Record.
Download the Windows 10 ISO from Microsoft. This is free and you do not need a product key.
Use the Boot Camp Assistant to download the Windows Support Software.
Mount the Windows 10 ISO and copy all the files to the flash drive.
Copy all the Windows Support Software files to the flash drive. The $WinPEDriver$, BootCamp and (Optional) AutoUnattend.xml files should appear in the root folder of the flash drive.


Update 3: Install the Boot Files and the Drivers


Boot From the USB flash drive. When the first window appears, enter a shift+F10 to open a Command Prompt window.
Use the diskpart command to determine the drive letters for the Windows WINSTALL volumes. (i.e. enter list volume) Here, I will assume the letters are C and D, respectively. Also, assign the EFI volume the drive letter S. See diskpart for more information.
Enter the following command to create the boot files. If needed, substitute your Windows drive letter for the letter C. See bcdboot for more information.

bcdboot C:\Windows /s S: /f UEFI

Add the drivers to the drivers store. Often this step can be omitted. If needed, substitute your Windows drive letter for the letter C and your WINSTALL drive letter for the letter D. See dism for more information.

dism /Image:C:\ /Add-Driver /Driver:D:\$WinPEDriver$ /Recurse  /ForceUnsigned



  You can ignore any error messages produced.

If you succeed in booting Windows, install the Windows Support Software.