Ted, we recently had a discussion on CD and DVD media and the life expectancy predictions were all over the map depending on who read what. Without getting into firm numbers, I've seen hard drives last a few months to lasting many years.
There are advantages to using a hard drive for backup but, first, what do you mean by "backup"? There is a difference between backup and archive. In the case of backup, the primary copy of the data will exist elsewhere. That means you will always have a minimum of TWO copies of the data. In the case of an archive, there may only be one copy on archive media.
One of the issues with backing up to disk is the finite space on the disk. Someone once said that if the backup drive is the same size as the main disk drive, it can always be used as a backup. This may not necessarily be true if you plan on keeping multiple generations (current backup + one that is a month old + one that is 3 months old + one a year old). The more generations you plan to keep, the more space will be taken up. You need to analyze your data to determine how many generations of that data would be kept. Say you are keeping backups of data that changes daily and a loss of 1 week would be unacceptable. Then there would not be much point keeping anything older than a week old as the data would be worthless. Keep in mind, this may not really be the case when you can restore two weeks back and then go through physical papers to recreate the missing data. If you figured out that only ONE copy of all of your data is all that is really needed, then you have very little to worry about because the odds of losing both the original data and the backup is small. However, if you will need to have access to a lot of older generations of the data, making several backups may be a good idea if they are all on seperate media as any media can suffer sudden death brought about by non-disk related things such as temperature problems (fan dies?), processor problem, fire, theft, etc.
If you really want to know what the manufacturer thinks is the lifetime of the disk, look for a statistic on the manufacturer's description of the drive called "Mean Time Before Failure" or "MTBF". There are different measuring units for this but it should give you an idea of how to compare drives. This, of course, assumes that the disk manufacturers all thoroughly tested their drives to come up with this number and there is some conformity in the testing. Still, this is a "mean time". Your mileage may vary (for better or worse).
You don't need to "retire" your drive arbitrarily. Many times a drive will give you a warning before they go such as data errors in some file or by the use of S.M.A.R.T. technology. You can alwas "push" the issue by running diagnostic tests (non-destructive) on the drive and see if small errors popup. I would not recommend this, though, because the tests could damage the drive more than you would expect.
Here are some alternatives to think about:
1) Periodically, burn your data to a DVD. Especially if you use archive grade, these backups could last awhile even though your current backup is still on the disk. If you don't use archive grade, the MTBF is about 5 years and some people reported having degradation after only three.
2) Consider a RAID system for your main data drive and/or the backup drive. If you don't know much about RAID DISK, it can easily be looked up. I would recommend something fast, like, RAID 1 for your primary data drive and a slower RAID 5 or 6 on your backup drives.
3) If there will be a "gap" between your last backup date and your current date for your data, consider keeping some paper copies for the short term (Quicken can print transaction reports, for example).
4) If any drive or fan starts to make unusual noises, that is the time for immediate repair or replacement. Waiting a few weeks is not a good option.
So, to answer your question directly, the MTBF statistic could be used as a guide but remember that a drive can fail at any time and this stat is only a mean/average figure. The best way to handle this is by planning what data needs to be backed up, how often the data changes and ask youself "If the drive (backup or main) dies in 5 minutes, how much trouble would you be in?" Use redundancy for critical data and consider environmental changes as a cause of problems almost more than just the drive "wearing out".
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