I came across some additional things that might prove useful.
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During my latest installation attempt the installer said the HDD had Zorin installed on it and offered the option to install Zorin alongside Zorin.
Yet when I try to run the system without the USB, it says there is no bootable device. That suggests to me there is something wrong with the booting process (and the boot partition if there is such a thing). -
Also, I used to have the option in the boot menu to boot from USB or from the harddrive. Somewhere along the way, I've lost the latter option and it now only allows me to boot from USB, another point towards something being wrong with the boot.
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When I tried the "Something else" option in the installer and told it to write to /dev/sda1 it threw an error I can't remember. (I.e. it refused to do it).
I'll see if booting from a USB burned with a different utility makes a difference.
Otherwise, I'll try to install a different Linux distro to get the harddrive sorted and see about replacing the other one with Zorin.
