MukZorinboot, you've implemented a good workaround to that installer flaw.
It is a variation of the Ubuntu bug forum's workaround(s) for that situation - when trying to install an Ubuntu-based Linux which uses the Ubiquity installer, to a separate USB drive, the created Linux drive will then only boot from that single computer.
This is a known flaw in the Ubuntu "Ubiquity" installer, it has been known for at least 8 years (within the small realm of Ubuntu tech folks and affected disgruntled users), but the Ubuntu tech teams did not feel it was significant enough to fix until 2023.
References:
Bug #1396379 installer uses first EFI system partition found
Bug #1396379 “installer uses first EFI system partition found ev...” : Bugs : ubiquity package : Ubuntu
Bug #1591352 “Ubiquity copies efi boot information to wrong ESP
Bug #1591352 “Ubiquity copies efi boot information to wrong ESP” : Bugs : ubiquity package : Ubuntu
Ubiquity mounts wrong ESP during install - (7-2016) - Ask Ubuntu
system installation - Ubiquity mounts wrong ESP during install - Ask Ubuntu
All of the workarounds, do something to hide the main computer's primary HDD/SSD drive or boot partitions from Ubiquity. Because the flawed Ubiquity installer doesn't notice or care what secondary/external drive you are installing Linux to, it just writes the GRUB boot records into the boot partition(s) of the Primary/First drive it finds - that being the internal HDD/SSD of the running computer.
Note, if one is installing Ubuntu-based Linux to a single physical internal drive, whether single or dual boot, it will correctly write the boot info to the single only internal drive. So most people don't encounter this bug.
It is really kind of sad that Ubuntu devs did not see fit to correct the flaw until 2023 (in their 9-month releases), so it didn't get fixed in the LTS until 24.04. With their new Flutter-code based installer one could now safely create an external drive with a live Linux install and the boot records would be properly written to the target external drive.
Note 2, other non-Ubiquity installers do not have that flaw, so Calamares and etc. work OK for external Linux installs.
This fix to the flawed Ubiquity installer of Ubuntu 16-22 (and possibly earlier), probably won't show up in Zorin until v 18 (just a guess on my part) since the bulk of the Zorin 17 is based on Ubuntu 22.04 LTS, which includes the flawed Ubiquity installer.
You can see this if you look at the package list. DistroWatch is a great resource for this kind of researching which distro's have flawed Ubiquity or not.
P.S. I did not test the various hide-the-boot-drive workarounds myself, as I was not in a position to dedicate a machine to that sort of testing. And being wary, I've still not tested it...