Thanks to the thread posted by Brian re. mounting ISO images and subsequent advice posted, I’ve solved the issue by mounting the USB to a mount point, in my case '/mnt/debian/‘.
Following that I edited ‘etc/apt/sources.list’ and commented out all lines and added the following : deb [trusted=yes check-valid-until=no] file:/mnt/debian/ buster contrib main non-free Then ran ‘apt update’ followed by 'apt install build-essential'. Thank you for the help, now I know for the future. As an aside ‘apt-cdrom’ did not work for me as posted on other forums / blogs. Jacques
