Stuck upgrading from Trisquel 7..9 to Trisquel 10 from dvd

On a Minifree X200 that started out on Trisquel 7.0, with an ext4 root and a btrfs user drive.

This is the machine whose operating system I don't trust because I never got to the end of:
https://trisquel.info/en/forum/auditing-installed-trisquel-9-against-install-cd

I booted Trisquel 10, split between a loopback mount of the iso on a thumb drive, and the iso image burnt onto a DVD.

I ran the install icon on the desktop.

I get to a loop between three pages of that app:

- Install (as superuser)
-- selected "Install alongside Trisquel GNU/Linux 9.0.2 Etonia

- Install Trisquel alongside ...
-- clicked through, without asking to change the sizes
-- "An error occurred while writing the changes to the storage devices. The rsize operation has been aborted."

- Installation type
-- never shows partitions or un-greys "Install now"

- Install (as superuser) reappears

Where do I look for the logs that give specific detail about what's going wrong?