Linux Mint doesn’t like the Ubuntu method of upgrading. They tell you all the bad things that can happen and why they think nuking your partition is a far better idea in their upgrade instructions.
In principal they may be right, but when things go wrong during the install they are far harder to recover from – as I’m discovering…
I seem to be running into the squashfs issue, but the solution listed in the post I found about it seems out of date. My USb stick was mounted as /cdrom/ and so I ran
cd /cdrom/ && md5sum --check md5sum.txt;
The results showed what I feared.
./casper/filesystem.squashfs: FAILED
Hunting around I eventually found where the locations were set (/usr/share/ubiquity/install.py), but short of editing that file it wasn’t clear to me where I could set an alternative path for the squashfs image to be stored. There’s likely a way to do it, but some web searches failed to turn it up (I did the searches from Mint Live by going direct to Google as the default search engine in Mint is DuckDuckGo which I find consistently hopeless).
My previous attempt at installing failed when the machine froze totally and so I’ve now rebooted and am trying again. If the same happens again I’ll have to seriously consider whether to carry on with Mint or whether to revert back to Ubuntu which despite having many issues was at least rock solid on this machine.