Verify your data
Although almost everyone understands the importance of backing up data, making sure the data was actually stored successfully often gets much less attention -- until there is a system failure and the backed up data turns out to be partially unreadable or worse.
While all backup programs perform some basic checks to verify the data, those checks may not be complete. It may be, for example, that the entire file tree is on the backup medium, but that some of the file lengths have been set to zero. It's a good idea to perform additional verifications on your data occasionally to make sure that it is actually completely backed up.
One simple way to do this is to occasionally pick a file, or even a complete directory, at random and restore it off the backup. A more complicated, but more thorough, variation is to restore to a second machine to make sure the system isn't just ignoring a corrupt backup.
Although verification is usually straightforward to do manually, several companies make software that makes verification easier. For example Veracity is a data integrity checker that handles a variety of jobs, including verification. The program also exists in a free version, Free Veracity for Linux and BSD systems.
Some companies, such as IBM, offer verification services that will check your data for you. Information for IBM's service for users of the NUMA-Q architecture is at: http://www.sequent.com/cs/backup_data.html
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.