Hi Dave ;-) Didn't you try with Uruk 2 Alpha recently ?
This comment however seems to confirm what you are experiencing, ieI can confirm that the Trisquel iso image, while written successfully, won't boot. The resulting usb stick is not bootable, at least in a UEFI environment.I tried Debian 9.01 Mate. It works as expected with the same dd command on the same usb device
https://trisquel.info/fr/forum/new-trisquel-8-alpha-images?page=1#comment-116979
