Boot Camp External Drive only booting to one computer Firstly, I apologize for duplicate posts -- I realized that I wasn't signed in to my account on StackExchange.

I followed these instructions (Is it possible to use Boot Camp with Windows 10 from an external HDD?) to install Windows 10 to my external SSD (Samsung T3). I followed Alex Lambert's guide (the one with most upvotes &amp; correct answer), and was able to install and boot Windows 10 successfully.

I ran to a problem today, as I was connecting the drive to two other Macs until I noticed that my drive was not being seen from the boot menu (option key), although I could see both of my drives (my drives are partitioned in half: 250GB for Windows (NTFS) and 250GB for Data for both OSes, Windows and OS X (exFAT)) in OS X on the other computer.

The computer I used to install was a MacBook Air Mid 2013 and the computers I used to test were two MacBook Pro Retinas made between 2014 and 2015.

Please help, apologies for the noob question (I'm new to this site).

Thanks.
The general rule is that the 2015 and newer Macs can no longer boot operating systems using the BIOS method. This is the method used in the installation instructions you refer to. Also, generally 2011 and older Macs can not boot Windows from external drives. Therefore, the installation method, you refer to, generally only works for the 2012 through 2014 model year Mac computers. 

The steps to determine if your Mac can BIOS boot operation systems is given below.


From the menu bar, select "About this Mac". The pop up window
should display the model year and screen size of your Mac. The pop
up window for my Mac is shown below. I have a  20-inch, Mid 2007
iMac.


Goto the Apple web site: Install Windows 7 and earlier on your Mac
using Boot Camp. If the "Boot Camp Support Software" is listed
for your model Mac, then your Mac can BIOS boot operating systems.



  But even if both computers qualify to BIOS boot Windows from external drives, you have the problem of the drivers. One of the steps, in the instruction you refer to, is to install the Boot Camp Support Software. This software installs the correct drives for your model Mac. If you then try to boot Windows using a different model Mac, the installed drivers will be wrong for the this Mac.