You likely need to disable secure boot in the BIOS (if you didn't already).
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Explaining Computers on YouTube has a great video on how to do this..
That is exactly how I have it on my system.
I can't help with the Linuxing or the external boot drive but you should honestly just run Windows 11 instead of 10. Framework doesn't officially support 10, and after the initial setup, I've noticed no downsides to it day to day
Oh really? Crap, I’ll have to upgrade then. I installed win10, and don’t see an automatic way to upgrade so I’ll have to dig around a little
Windows Update should offer you the upgrade. If it does not, you can go to the site for installing Windows 11 (https://www.microsoft.com/software-download/windows11) and upgrade there.
I had to do a fresh install cause win10 doesn’t support the tpm chip :(
Yes you can, Linux doesn't care. Do note that secure boot is a thing and that might mess you up
Glad you asked.
Your biggest concern is if you overwrite the windows bootloader with the Grub2. What will happen then is that windows won't boot without the external Linux Drive.
If you're wanting the most braindead way to avoid that, go ahead and remove your Windows SSD before installing Ubuntu. There are better ways if you're more competent in Linux (unlike me)
There might be some bootloader shenanigans required. Like putting GRUB on the internal drive, then installing Windows, then putting Linux on the external drive.