Hello,I found that the problem is that check-missing-firmware stops scanning (through mountmedia) as soon as it could mount one partition.
Given that my USB is on /dev/sdc1, and that mountmedia could mount /dev/sda1 to /media (ESP partition which is FAT), check-missing-firmware searched in /dev/sda1 and then stopped searching firmwares on any other partition.
More details in: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1029543#45 Regards
