With current pricing, hard disk drives are incredibly cheap. Even high-performance SCSI drives are becoming less and less expensive. Most of them even come with three or five-year warranties. So the cost of replacing the physical drive is not a huge financial burden. However, there's another factor in play: your data.

Even assuming you are performing good, reliable server backups and that those backups are intact, you still have the downtime associated with rebuilding the server. In order to restore from that backup, you have to install the new drive, format it, re-install the operating system, install service packs, install all relevant video, tape, SCSI drivers, install the backup software, and then restore from the tape. You run the risk that certain key information, such as the Windows NT Registry or the Windows 2000 System State Information, was not properly backed up. Some databases get backed up in an inconsistent state and must be repaired. Or, if the settings aren't just right, those databases might not be backed up at all.

A server rebuild, even with a backup, is generally a three to four-hour process if everything goes smoothly, during which that server -- and in many cases, an entire department or more -- is down for the count. On top of that, you could easily lose as much as a full day's worth of data.

On a workstation in a network where the policy is to store all created files on the server, the loss may not be as severe. However, it is usually not cost-effective to install a backup device on each workstation, and network backups are often slow and difficult. Even assuming no irreplacable data is stored on a workstation, you still have the time involved in replacing the drive, re-installing the operating system and all drivers, installing all the applications, and re-customizing the system for your network and for the user's needs. How much time will this require? How long will the user be unable to do his or her work?

There are basically two types of RAID: mirroring and striping. Mirroring involves simply writing the same information to two drives at once. They become mirror images of one another. If one fails, the system can continue running on the other one until the failed drive can be replaced. Striping is more complex; the information is written across multiple drives in such a way that any single drive in the array can fail, and the system can continue running until the failed drive can be replaced. In both cases, the drives must be at least approximately the same size -- whichever drive is smallest determines the size of the array.

Mirroring is the simplest technology. It uses two and only two drives per 'mirror set'. You buy two drives and get the storage space of one of them. For example, if you buy two twenty-gigabyte drives, you get twenty gigabytes of fault-tolerant storage. However, you can boot off of a software-mirrored drive with no real issues. This technology is also referred to as RAID-1.

Striping is more complex. It uses from three to thirty-two drives to form a 'stripe set'. One of the drives is designated to store parity information used to reconstruct the information on the other drives. If the parity drive is lost, all data is still present. If one of the other drives is lost, then the parity drive's data can be used to reconstruct what must have been on the failed drive. You can't boot off of a software-striped drive, however. Since one drive is used for parity information, you effectively lose the storage space of that one drive. If you have five twenty-gigabyte drives in an array, you get effectively 80 gigabytes of fault-tolerant storage. This technology is also referred to as RAID-5

Hardware RAID is more robust, since it doesn't require the operating system's intervention. Everything is managed directly by the card. You can always boot off of a hardware RAID array, whether it be a stripe-set or a mirror. Even operating systems such as Windows NT Workstation, Windows 2000 Professional, or Windows 95/98/ME, can use a hardware RAID card, even though they don't support RAID in software. However, Hardware RAID cards typically start at approximately $500 US for a SCSI model, and $100 for an IDE RAID model.

Software RAID functions without any additional hardware. Typically, only server operating systems support it, such as Windows NT Server, Windows 2000 Server, or Linux. Two problems arise, however. First, since most systems support a maximum of 4 IDE devices, one of which is typically the CD-ROM, implementing software IDE stripe-sets is typically not feasible. You can only do mirroring simply because you can't install enough drives. Further, while many OS's permit you to boot off of a mirrored drive, you usually cannot boot from a striped array. IDE is also poorly suited to the demands of mirroring or striping, since it does not handle multiple simultaneous reads and writes as well as SCSI does. Finally, when implementing software RAID, keep in mind that you will see a performance hit on the system due to the demands of keeping the drives synchronized.

The primary situation where this arises is RAID 5 implemented via software. Companies want to use RAID 5 because it is faster than mirroring, and gives you more space per amount of money spent. However, if it's implemented in software, two more drives must be added to store the system and boot partitions. Once the system boots, it can then access the data in the RAID 5 array.