Customize error message "Unable to find a medium containing a live file system"
The solution is to use the "External Hard Disk" entry of the Boot Loader (when it's GRUB). See https://tails.boum.org/doc/advanced_topics/external_hard_disk/.
But maybe we can customize this error message to explain that and point to the doc?
That would help people with problematic USB stick that are not flagged as removable, like #19605 (closed).