Hi, I was using Zorin for the last 18 months and was quite happy with it!
After receiving notifications that my disc space was running short and my email program didn't work anymore I tried deinstalling the mail program.
Now the system doesn't boot at all. Maybe deleting the package caused the disc space to completely run out?
Now, before doing anything else, I would like to save my data to an external HD.
what's the easiest way to do this without access to zorin? Maybe via a bootable USB drive? I have one with gparted installed but not sure if it works with this and how.
The easiest way to backup your files using an external HD or SSD is to create a bootable USB with whatever Linux operating system you want. I'd recommend doing this with Ubuntu instead of Zorin, grab a USB and flash the OS using Etcher. Once that's done, you can boot from the USB through your UEFI or BIOS menu. Once the checksums complete and you're running in "test" mode, you should be able to access the files on your system and back them up to your HD or SSD using Casper.
Rather than explaining all of that here, I'll provide a YouTube link that will walk you through the casper.rw setup process, which can be watched here:
If you still have your ZorinOS installation USB, boot that and choose "Try Zorin" mode.
From that, use Zorin>Utilities>Disks app to see if your disk particularly /home partition is visible.
Connect an external drive where you wish to backup your files and check that is visible.
Then either
copy the contents of your home partition to the external drive
or
use Zorin>Utilities>Backups app (dejadup) to backup your home partition to the external drive