I am afraid Zorin has inherited a bug of the Ubuntu 22.04 installer. I wanted to install Zorin on an external SSD. My laptop has 2 internal SSD, one with Windows 11 and the other SSD with Windows 10 and Ubuntu 25.10 in dual boot. When I wanted to install Zoring on the external SSD, I carefully chose the manual partitionning mode. I created EFI and \ partitions for Zorin. I checked bootloader to be on the external SSD. Result was a disaster: bootloader was not installed on the external SSD but on the internal second SSD, conflicting with Ubuntu which could no longer be booted. I tried twice, same result. I had to reinstall Ubuntu. Advice to Zorin: use the new Ubuntu installer (from 25.04 or 25.10) or a customised Calamares installer. Ensure that the bootloader install in its dedicated folder, not one called 'ubuntu' as it is now.
I realize that this may not be a valid solution for you (and it's just a test that if you could do, would maybe be a workaround for now) but if you could remove the two internal SSDs, does it then function correctly on installation? I must admit I've not tried to install to an external drive outside of my 2 internal SSDs, so I haven't run into a bug like you're describing. I might have to give it a try later if I get some time.
Thanks! That would certainly solve it as there would be only one EFI partition. But this is a Legion Laptop and I am lazy to open it and remove those SSDs...
Another option rather than removing the drives is you could go into your BIOS and disable the slots you don't want to use, while you do the install on the external SSD. I'm not sure if your laptop would have those options or not, but it might be worth a look as well.
Welcome to the Forum!
Do You talk about Zorin 17 or 18?
I found a solution: in Gparted I changed flags for the Ubuntu EFI partition. I unselected ESP BOOT flags and put it as hidden. When I installed Zorin on the external SSD, the bootloader correctly installed on it and the Ubuntu bootloader/grub was left intact.
Thank you! Zorin 18.
When this solves the Issue for You, I marked it as Solution.