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Server Security: Keeping Your Data Safe Page 5
By Karl Magsig
Tape backup units are not the only solution for backing up data. CD writeable and re-writeable drives are available for use as backup devices as well. In general, CDs are faster than tape backup drives, but they normally cannot hold as much data as a tape can. CDRs (writeable CDs) are also one-time-use devices, a CDRW (CD re-writeable) drive is needed to reuse CD media.
Another popular method of backing up data is "mirroring" the hard drive. This is done by installing a second hard drive of equal or greater capacity to the original and, through the use of special software, creating a mirror image of the original. This way, if a hard drive fails for some reason, there is an exact copy of all of the data. In these cases, it is a simple task to switch the secondary hard drive to the primary and keep running as if nothing happened.
Computer hardware manufacturers have also created a redundant hard drive system called a "RAID array." In such a configuration, data is spanned across several separate hard drives running in the server, with each hard drive storing only a portion of the data. If one of the hard drives should fail, the remaining drives have enough information stored on them to piece together and replace the data that was lost when the other drive went down (there are normally four or more drives in a RAID configuration.) In this way, server operation and data access is not interrupted. If they are interrupted, it is only briefly, while the remaining hard drives pick up the slack and rebuild the missing data. Then, as soon as possible, the server is taken down, and the bad hard drive is replaced. When the server comes back up, the remaining hard drives use the data stored on them to load the information onto the new drive.