I tried Lab 9.3 using Ubuntu 14.04 and CentOS 7 and encountered problems with the following commands and have given suggested workarounds:
1. sudo mkfs.vfat /dev/loop1p3 - workaround - sudo mkfs.ext4 /dev/loop1p3.
:~$ sudo mkfs.vfat /dev/loop1p3
mkfs.fat 3.0.26 (2014-03-07)
unable to get drive geometry, using default 255/63
2. sudo losetup -d imagefile - workaround - sudo losetup -d /dev/loop1.
Interestingly, the command - sudo losetup -l - gave me sufficient information to suggest a workaround (works in CentOS 7 but not Ubuntu 14.04).