Windows external hard drive not detected in Startup Disk List

Question

I am trying to set up an external hard drive with a copy of Windows 10 that I can boot from my MacBook Pro (Early 2015). I used the procedure posted by Alex Lambert in the following thread:

Is it possible to use Boot Camp with Windows 10 from an external HDD?

Following his procedure, I never received any error messages, but when I reboot my MacBook and hold down the option key, the only bootable hard drive listed is my "Macintosh HD". If I open System preferences-->Startup Disk, it finds my external hard drive as a Windows disk, but if I try to select it, then I get the error: "You can't change the startup disk to the selected disk. The bless tool was unable to set the current boot disk."

How can I check what is missing on the hard drive that keeps it from showing up in the startup disk selection?

Answer 1

This procedure will not work for your 2015 Mac. Step 6.2.5 states: "Run: clean". The next step (6.2.6) states: "Run: create partition primary". This indicates the external drive is using the Master Boot Record (MBR) partition scheme. The 2015 and later Macs can no longer boot operating systems using this scheme. The question you refer to involved a 2014 Mac.

This procedure you reference requires you to download the "Windows 7 Automated Installation Kit" in step 3. Later, step 6.2.4 uses the imagex command from this kit. This command has been deprecated and should not be used with Windows 10. The replacement command is dism.exe, which is included in the Windows 10 installation iso. So steps 3 and 5.3 are unnecessary.

This procedure may be adapted for your model Mac. One change would be that the external drive needs to be using the GUID Partition Table (GPT) scheme. This is probably not the only change needed. The accepted answer to the question "Unable to boot Windows 10 from Bootcamp external HDD" documents another possible change that may be needed.