Yes it is certainly possible - I do it and have no issue with either drivers or activation. The external install works on all my Macs with the exception of a 2006 MacbookPro2,1 which will not boot anything apart from OS X from an external drive.
You don't need to do anything special - once you have a bootable drive just plug it into another Mac/PC and it will generally work - Windows is very forgiving about hardware when it comes to booting these days.
Note that Windows 10 activation is tied to the Mac/PC hardware that you boot it from not the installation on the external disk. All my Macs have digital licenses for Windows so my external Windows activates whichever I boot it on. If you boot on a Mac/PC without a digital license for that edition of Windows then it will run un-activated until you either buy a license or boot from a Mac/PC with a license.
This question isn't really Mac specific but the first step is to install Windows to the external drive as normal. There are many methods and guides to do this. I did it manually as I already had other OSs on my external drive and I find it quicker.
The Microsoft recommended partition layout is described here but it doesn't much matter as long as you have a EFI partition and a Microsoft basic data partition. I partitioned the external drive using gdisk like this using the existing EFI partition (number 1) for Windows boot files and then created partition 4 for Windows as that is where I had space.
PS C:\Users\Hali\Downloads\gdisk-windows-1.0.5> .\gdisk64.exe 1:
GPT fdisk (gdisk) version 1.0.5
Partition table scan:
BSD: not present
APM: not present
Found valid GPT with protective MBR; using GPT.
Command (? for help): p
Disk 1:: 488397168 sectors, 232.9 GiB
Sector size (logical): 512 bytes
Disk identifier (GUID): 1EE267FB-E2ED-4704-8B4A-635AA0134B7D
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 488397134
Partitions will be aligned on 8-sector boundaries
Total free space is 1698070 sectors (829.1 MiB)
Number Start (sector) End (sector) Size Code Name
1 2048 411647 200.0 MiB EF00 ESP
2 411648 149356231 71.0 GiB AF00 SamsungHFS
3 149618688 247205343 46.5 GiB AF0A Mojave
4 247205344 413563199 79.3 GiB 0700 SamsungNTFS
5 413563200 465992007 25.0 GiB 0700 exFAT
6 465992008 486963534 10.0 GiB 8300 LinuxBackup
Command (? for help):
Next Windows was applied from the ISO to the NTFS partition using DISM and the Windows bootloader to (external drive) EFI partition using
bcdboot. This was done from Windows running on my internal Boot Camp partition. The Microsoft steps for doing this are here or you can look at this tutorial. You may then want to apply Boot Camp drivers at this point as your Macs are recent but I installed them after first boot.
If you don't have a Boot Camp partition you could use a separate Windows PC to do it or install Windows on VirtualBox, mount the external disk as a raw disk and then apply from there or use some other method as described in the links above.
Once you've installed Windows to your external drive (however you did it) you can then boot from it.
If you installed Windows 10 the activation is normally stored as a digital license tied to the hardware - it is not tied to the installation. Therefore if your Boot Camp partition has a valid license then the first time you boot from Windows on your external drive it will activate as soon as you connect to internet. If your Mac doesn't have a license then it will run un-activated indefinitely with some limitations. This is exactly the same way as activation works when dual booting on any PC.
- Windows running from SamsungNTFS partition on external SSD on a 2013 i5 MBP.
If you want to boot from another Mac just hold option and pick EFI boot on the external drive.
When booting a different Mac/PC you'll see a message "Getting devices ready.." during boot but this takes only a few seconds and you don't need to manually install drivers - it is automatic. This is the same Windows instance running on another Mac - you can see the drivers used in the previous boot (CPU/graphics etc) are still there just not active.
- Windows running from SamsungNTFS partition on external SSD on a 2014 i7 MBP.
Activation after swapping disk
If the second Mac has a valid digital license then 90% of the time it will activate automatically after a minute or two. If it doesn't (for me about 10% of the time I swap it doesn't) you'll see this message:
If you click on "Troubleshoot" and the hardware you are booting on has a valid digital license it will then activate.