Information stored on the hard drive of a computer is recorded as a series of magnetic impulses; it cannot be read, used, or manipulated without the hardware that stores it. Physical damage to the drive and other types of system corruption, including viruses, can easily undermine the integrity of the information stored on a drive by scrambling or damaging the drive's storage system. Ensuring stable storage in a computer means constructing an information storage system that is guaranteed against immediate errors following a write operation, which adds to or replaces stored data. Basic commercial hard drives do not qualify as stable storage on their own; however, with software and configuration tools, they can fulfill the demands of stable storage.
In order to be considered stable storage, following a write procedure — in which information is saved to the disk — a drive must be able to immediately read back the exact information that was just written, without errors. This explains why commercial hard drives fail as stable storage: there is always the possibility that a drive will return an error message following any specific write operation. Some techniques exist to turn commercial hard drives into stable storage devices, though.
Increasing the stability of commercial hard drives is possible through software management techniques. One software management technique that applies is forcing the drive to write to two separate places on the disk for each write operation, providing redundancy. While one of those places might return an error, the other is statistically unlikely to do the same, resulting in a higher level of stable storage.
Another technique is creating a Redundant Array of Inexpensive Disks, or RAID for short. A RAID1 volume is a redundant volume technique known as "Mirroring." It uses two hard drives working in tandem. When a write operation takes place, the same information is written to both drives at the same time. This provides a real-time backup solution for all data on the drive, making it a more secure form of stable storage.
There are downsides to these techniques. The first is economy; in either case, the effective size of the hard drive is halved, as information is constantly being duplicated for backup purposes. The second is cost; with essentially half-sized drives, storage expenses can become considerable depending on the amount of data being saved.
