How to boot external disk with Linux Mint on UEFI Windows 10

Feb 4, 2019
Hello, dear community.
I want to perform a simple task, i want to boot Linux Mint from external SSD where it is alrdy installed.

I have preinstalled windows 10 in UEFI/GPT on HP notebook.
And I have installed Linux mint on external SSD disk probably in MBR/legacy that i was using before.
I realy dont want to reinstall or instal new linux to dual boot on notebook.
I just want to use both systems that i alrdy installed, bcs i have there a lot of installed aplications and data. My imagination was just to plug-in external disk and then boot linux when needed.

Secure Boot is disabled and there is no option for booting external device.
I tried to change UEFI to legacy in bios but there was warning :"Changing this setting may make the system unable to boot the os windows 10". So i rather did nothing.

Q: Changing this setting can corrupt instalation of windows where i cant go back?
I was not sure if i can just change UEFI to Legacy for booting Linux and then go back to UEFI and boot windows.

Thank you for reading and some sugestions.
Sincerely Vladimír.
Vlad, you have three options:
- take out existing HDD/SSD from your laptop, switch over to Legacy, and see whether you can boot off your external HDD;
- make recovery Windows 10 bootable media (DVD/USB), and bite the bullet by performing above, back and forth, then repairing (or restoring) Windows installation;
- reinstall your Mint onto that same laptop, with UEFI/GPT scheme.

Feb 4, 2019
Taking out HDD is not an option right now, my notebook is still in varancy and sealed. And I dont see reinstall of OS as a solution.

But I am thinking about other option: changing Windows to MBR and change bios boot to legacy, it should be possible with easus app.

Q: And I still would like to know if changing boot mode to legacy is irreversible in a way where uefi windows cant be booted again without recovery.
If someone tried this or knows the answer please let me know.

Thank you for your answers and suggested options.