Hi Aravisian, hope you had a good sleep after a long day? I finally got the 16 Pro to boot from the USB but had to enable Legacy ROM in order to see the USB drive. It's still in UEFI mode but has the option to allow Legacy ROM, without that it doesn't see the USB drive as a bootable option. I think it may be a Dell BIOS problem as you hinted at, though I'm on the latest version.
Anyroad, the install routine still doesn't see my Win 10 installation. I've disabled FastBoot, SecureBoot, ShinyBoot, DullBoot and TwoHoots. I had to revert to the Balena Etcher to flash the USB drive as when flashed with Rufus nothing happens, after boot to USB, except a blank screen.
I looked at the 'Something Else' option and printed out the manual partitioning guide that the Zorin guys emailed to me, it's from Ubuntu, except the pictures won't print out.
What concerns me is that if the Zorin install routine doesn't detect my Win 10 installation, why would the Grub (is that correct?) loader see it once I'm done faffing around with bizarrely named Linux partitions and having sphincter palpitations when I hit restart.......
I've ordered a new external 1Tb USB hard drive solely to do a full Windows backup, separate from the ones I already have. So I may wait until tomorrow until the drive comes and I've done that.