The novice asked the backup master which files he should backup.
The master said: "Even as a shepherd watches over all the sheep in his flock, and the lioness watches over all her cubs, so must you backup every file in your care, no matter how lowly. For even the smallest file can take days to recreate."
The novice said: "I will save my working files, but not my system and application files, as they can be always be reinstalled from their distribution disks."
The master made no reply.
The next day, the novice's disk crashed. Three days later, the novice was still reinstalling software.