I'm curious if anyone has discovered any new gotchas with doing this. I haven't done it yet myself but thought I'd ask before going rusty venture on it 
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Bootcamp on external storage on T2 Macs?
- Thread starter spheris
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I am running Windows 10 on MacBook Pro 15 in 2018 from an external TB3 storage (Akitio Node Lite + Samsung 970 EVO NVMe). You do need to allow external boot from "Startup Security Utility" under Recovery Mode.
I have eGPU plugged into the left side TB3 port and Elgato TB3 dock plugged into the right side TB3 port. The Akitio Node Lite is connected to Elgato TB3 dock as daisy chain. Samsung 907 EVO NVMe over TB3 does 2,500 MB/s sequential read and 1,800 MB/s sequential write on my benchmark using Samsung Magician.
If you you use TB3 external cases rather than USB 3.1 gen 1 or gen 2 external case, you don't even have to deal with Win2GO route for installation.
I have eGPU plugged into the left side TB3 port and Elgato TB3 dock plugged into the right side TB3 port. The Akitio Node Lite is connected to Elgato TB3 dock as daisy chain. Samsung 907 EVO NVMe over TB3 does 2,500 MB/s sequential read and 1,800 MB/s sequential write on my benchmark using Samsung Magician.
If you you use TB3 external cases rather than USB 3.1 gen 1 or gen 2 external case, you don't even have to deal with Win2GO route for installation.
Did you direct install from dvd or other? If you could give a little more detail it would be appreciated/
Thanks
Yes. I installed Windows 10 directly without using BootCamp. Windows treats TB3 drives as internal drives for the purpose of installation, unlike USB drives which it treats as external drives.
You have to first boot into Mac "Startup Security Utility" under Recovery (Command-R during boot). Select Utilities, then "Startup Security Utility" from menu. Authenticate with macOS password and admin password. You have to pick Medium Security or No Security (if medium security didn't work). Under External Boot, you have to pick "allow booting from external media" option.
You have to install Windows 10 1803 or older. Windows 10 1809 will fail to complete installation (missing drivers or something). Simplest is to make Windows installation USB thumb drive. Unfortunately MS Windows website will create Windows 10 1809 version USB installation drive. So you will have to download older Windows 10 1803 or other older Windows ISO (Windows 1703 works best) and create your own USB installation drive. You can use Rufus, WinToUSB, or many other programs to make USB installation drive.
Simply boot to Startup Option with "alt" and install from the USB installation keydrive. After installation, update with BootCamp drivers. After that just let Windows Update updated Windows version.
Windows 10 1809 update maybe problem for certain Mac.
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Thank you
I really appreciate the reply.
No Joy, Went straight to a boot loop to the recovery mode. Possibly a difference in the way the T2 works on the iMac pro Vs MacBook Pro
I really appreciate the reply.
No Joy, Went straight to a boot loop to the recovery mode. Possibly a difference in the way the T2 works on the iMac pro Vs MacBook Pro
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Also experiencing the exact same thing with my 2018 Mini, did you investigate further or give up? I don't really want to sacrifice internal disk space just for occasionally booting into Windows.
Managed to get this working on my 2018 Air - although it required the use of my 2011 iMac as I couldn't start the first stage direct from the Air. This is through USB3, not TB3 too.
I based it on this method: How to use Boot Camp on an External Drive) - I had to use VirtualBox on my iMac 2011 as I got permission errors when trying to mount the disk (possible Mojave specific?). A few other quirks are covered here: Medium @SvenKirsimae
Someone has already mentioned it, but stick with Win10 1803. 1809 changed a driver which breaks booting on an external.
I based it on this method: How to use Boot Camp on an External Drive) - I had to use VirtualBox on my iMac 2011 as I got permission errors when trying to mount the disk (possible Mojave specific?). A few other quirks are covered here: Medium @SvenKirsimae
Someone has already mentioned it, but stick with Win10 1803. 1809 changed a driver which breaks booting on an external.