You posted in a comment: "Currently it appears to be mission impossible." Well, here is an example question, where an user had Windows installed to BIOS boot from an external drive on a 2012 Mac.
I believe your model Mac is suppose to BIOS boot Windows 10. You have a BIOS booting Windows 10 installed on the internal drive. You should probably install Windows 10 to BIOS boot from an external drive. The instructions given in your link are for installing an EFI booting Windows on an external drive.
BIOS booting Windows installations can only access the first 2.2 TB of the installation drive, when the sector size is 512 bytes. The may be a problem since your external HDD is 5 TB in size.
The Boot Camp Assistant should be able to create a USB flash drive Windows 10 installer. This installer should be bootable on your Mac. You could use this installer to try and install Windows 10 on the external drives. These drives will need to be either MBR partitioned or hybrid partitioned like the internal drive.
While the Boot Camp Assistant can create a USB flash drive Windows 10 installer, the Boot Camp Assistant probably can not be used to install Windows 10 on an external drive. This usually requires manual installation using the Windows CLI.
UPDATE
User Cordvision posted the question Big Sur: Booting Win10 from SSD and Disable Internal Drive. This question documents how a firmware update included with macOS has resulted in problems BIOS booting Windows from external drives. I found that on my 2013 iMac the Mac Startup Manager will not always display an icon or the correct icon for an external BIOS bootable drive. Also, when choosing to BIOS boot from an external drive, the Mac may instead try to BIOS boot from a different drive. To overcome this limitation, I installed the rEFInd Boot Manager. This is a free product.
Installing BIOS Booting Windows 10 on USB HDD Using 2013 iMac
These instructions will probably only work with 2012 though 2014 Macs.
I do not have the same hardware as you have. This is as close as I can get. I am also using a wireless apple keyboard and mouse. I would recommend using a wired Apple keyboard and mouse during the installation. If you have any questions about what a step does, you probably should first ask for an better explanation.
I have Catalina, High Sierra and a BIOS booting Windows 10 installed on the internal HDD. Windows 10 is installed in the partition with the identifier disk0s3. The external USB drive does not have any operating systems installed. I will be installing a BIOS booting Window 10 the partition with the identifier disk2s3. Currently, this partition contains an ExFAT formatted volume.
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_APFS Container disk1 898.0 GB disk0s2
3: Microsoft Basic Data BOOTCAMP 91.5 GB disk0s3
4: Microsoft Basic Data MyExFAT 10.5 GB disk0s4
/dev/disk1 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +898.0 GB disk1
Physical Store disk0s2
1: APFS Volume Muskie 28.1 GB disk1s1
2: APFS Volume Preboot 118.9 MB disk1s2
3: APFS Volume Recovery 1.0 GB disk1s3
4: APFS Volume VM 8.6 GB disk1s4
5: APFS Volume Snapper2 - Data 21.6 GB disk1s7
6: APFS Volume Snapper2 11.3 GB disk1s8
7: APFS Volume Ron2Pasport 440.1 GB disk1s9
8: APFS Volume FromTuna 35.3 GB disk1s11
9: APFS Volume DavidAnderson 71.8 GB disk1s12
/dev/disk2 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *400.1 GB disk2
1: EFI EFI 209.7 MB disk2s1
2: Apple_APFS Container disk3 340.0 GB disk2s2
3: Microsoft Basic Data BootCamp 59.9 GB disk2s3
/dev/disk3 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +340.0 GB disk3
Physical Store disk2s2
1: APFS Volume RonsStuff 19.0 GB disk3s1
2: APFS Volume SnapperBackup-Summer19 186.5 GB disk3s2
If you choose to install a BIOS booting Windows on the 5 TB USB drive, then you will need to create an ExFAT formatted volume some where within the first 2.2 TB of the drive. Assuming disk5 is the identifier for your 5 TB drive, you could enter the command below to create a 500 GB partition for Windows.
diskutil partitiondisk disk5 gpt exfat BootCamp 500G jhfs+ seagate R
The steps I used to install Windows 10 to an external HDD are given below.
Download and install gdisk from the SourceForge website GPT fdisk. This is a free product. First, download to your Downloads folder. Next, enter the command below in a Terminal application window to remove the quarantine.
xattr -c ~/Downloads/gdisk*.pkg
Finally, use the Finder application to open the downloaded file and install gdisk.
Use the gdisk command to insure the USB drive is hybrid partitioned. Below is what I entered in a Terminal application window. Note: According to Wikipedia, APFS partitions do not have a defined MBR hex code, so I choose to use the hex code da.
sudo gdisk /dev/disk2
r
h
2 3
y
da
n
07
y
n
w
y
Below is the output, when I entered the above.
davidanderson@Snapper ~ % sudo gdisk /dev/disk2
GPT fdisk (gdisk) version 1.0.7
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 hybrid 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
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): da
Set the bootable flag? (Y/N): n
Creating entry for GPT partition #3 (MBR partition #3)
Enter an MBR hex code (default 07): 07
Set the bootable flag? (Y/N): y
Unused partition space(s) found. Use one to protect more partitions? (Y/N): n
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/disk2.
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.
If you used your 5 TB USB drive with the partitioning I described above, then you would not be able to include the jhfs+ formatted partition, because this partition exists beyond the 2.2 TB limit. In this case, you would enter the commands given below.
sudo gdisk /dev/disk5
r
h
2
y
07
y
n
w
y
Use the Boot Camp Assistant application to create a Windows 10 installer flash drive. Below is an image of my selection from Boot Camp Assistant included with Catalina. Do not use the Boot Camp Assistant to install Windows 10. Only create the flash drive installer from the download Windows 10 ISO file. This file can be download from this Windows website. The file I downloaded was named Win10_20H2_English_x64.iso.
When finished, check to see if the root folder on the flash drive contains the file named AutoUnattend.xml. If found, change the name to NoAutoUnattend.xml.
Download and install the rEFInd Boot Manager from the SourceForge rEFInd website. Here I will assume the downloaded zip file was expanded to the folder refind-bin-0.13.2. If you have a different folder, then make the appropriate substitutions. Note: The diskutil commands may state the volume is already unmounted and the refund-install script will produce an error message with respect to either the sed or bless command.
Note: disk0s1 is the EFI partition on my Mac's internal drive. If this is not true in your case, then make the appropriate substitutions. In addition, you can use the EFI partition on the drive where you are installing Windows 10, as an alternative location for rEFInd.
cd ~/Downloads
xattr -rc refind-bin-0.13.2
cd refind-bin-0.13.2
diskutil unmount /dev/disk0s1
./refind-install --usedefault /dev/disk0s1
echo "scanfor biosexternal" >> /Volumes/EFI/EFI/BOOT/refind.conf
diskutil unmount /dev/disk0s1
Shutdown the Mac. Remove all external drive except the Windows 10 installer flash drive. Start the Mac and immediately hold down the option key until the Mac Startup Manager icons appear. Highlight the external drive icon labeled EFI Boot, then select the arrow under this icon. This will boot to Mac from the Windows 10 installer flash drive. Below is a image similar to the external drive icon you will see.
The first window to appear should be similar to the one shown below. At this point, you can plug in the drive where Windows will be installed.
Proceed as one would for a GUI installation of Windows. When a window similar to the one shown below appears, press the shift+F10 key combination.
Note: If you press the shift+F10 key combination before the image below appears, then the Windows installer may not have loaded drivers from the Windows Support Software.
The result should be the appearance of the Command Prompt window shown below.
Enter the command given below to determine the drive letters.
echo list vol | diskpart
In this example, drives d: and f: were determined to be the ExFAT volume on the Windows installer flash drive and the ExFAT volume where Windows will be installed, respectively. If your drive letters are different, then make the appropriate substitutions in the remaining steps.
If necessary, enter the command below to get the names of the available Windows installations.
dism /get-imageinfo /imagefile:d:\sources\install.wim
In this example, the name Windows 10 Pro will be used. If you choose a different name, then make the appropriate substitution in the following step.
Enter the following commands to begin installing a BIOS booting Windows 10.
Note: the first dism command will take a long time to complete. You should ignore any error messages from the second dism command.
format f: /q /v:BootCamp /fs:ntfs
dism /apply-image /imagefile:d:\sources\install.wim /name:"Windows 10 Pro" /applydir:f:\ /checkintegrity
dism /image:f:\ /add-driver /driver:d:\$WinPEDriver$ /recurse /forceunsigned
bcdboot f:\windows /l en-us /s f: /f bios
bootsect /nt60 f: /mbr
exit
When finished , close the Windows Setup window by clicking on the red box with the X mark. In the next window, select Repair your computer, then Turn off your PC. After the Mac turns off, remove the flash drive.
Start the Mac and immediately hold down the option key until the Mac Startup Manager icons appear. Highlight the internal drive icon labeled EFI Boot. Below is a image similar to the internal drive icon you will see.
However, if you installed rEFInd to an external drive, then you should instead highlight the external drive icon labeled EFI Boot. Below is a image similar to the external drive icon you will see.
Next, hold down the control to change the straight arrow to a circular arrow. Select this circular arrow to boot the Mac to the rEFInd Boot Manager. With the icon shown below highlighted on the rEFInd display, press the return key. If the icon does not appear, then press the esc key to refresh.
Finish installing Windows 10. The computer will automatically reboot to rEFInd, then Windows 10 on the external drive. When finished installing Windows 10, plug in the Windows 10 installer flash drive and execute the Setup application in the BootCamp folder on the flash drive. Afterwards, eject and remove the Windows 10 installer flash drive before restarting the Mac.
There should be an Apple Software Update application installed on your Mac. Execute this application to get updates from Apple. I would recommend installing any updates to Apple Software Update first.
Finally, check with Windows Update for any additional (optional) driver and other software updates.
Additional References
Man page of gdisk
Man page of refind-install
DISM Image Management Command-Line Options
DISM Driver Servicing (.inf) Command-Line Options
BCDBoot Command-Line Options
Bootsect Command-Line Options
BIOS/MBR-based hard drive partitions
The rEFInd Boot Manager
The rEFInd Boot Manager: Configuring the Boot Manager
The rEFInd Boot Manager: Using rEFInd
blesscommands given in your question do not do this. Theblesscommands given in your question create the files named.disk_labeland.disk_label_2x. To change the default to boot, theblesscommand would at least need to include the--setBootoption. Also, I should mention thesudo systemsetup -setstartupdisk pathcommand has replaced the bless command for the purpose of setting the startup disk.Administrator: Command Promptwindow and post the output from the commandecho list disk | diskpart?