try this (like in my method 1 given above) "lazy forced" umount, which worked well for me: umount -l -f /media/iso mount -oloop -tiso9660 <next-iso-file> /media/iso
try this (like in my method 1 given above) "lazy forced" umount, which worked well for me: umount -l -f /media/iso mount -oloop -tiso9660 <next-iso-file> /media/iso