6.3.17 Disk Only Storage  RAID arrays are scalable within the limits of the system, however individual HDDs are infinitely scalable by simply adding more drives. Since the introduction of the IBM 3340 HDD, storage capacity has increased rapidly, almost exponentially, while costs have fallen. These changes, linked with an improvement in reliability, have led some to suggest that HDDs could be used as both the primary storage system and the back up copy. There are three difficulties associated with this approach: Firstly, hard disk life is estimated in terms of usage- time, that is the number of hours of operation. There has been no testing of the life of an infrequently used HDD. Secondly, having data on different types of media is advantageous as it spreads the risk of failure. Therefore the approach should be considered very cautiously. Finally, there is no way of monitoring the condition of the hard disk on the shelf without turning it on at regular intervals and thereby compromising the advantage gained by having the disk turned off (see section 6.3.18 below, Monitoring of Hard Disk Media). Multiple carriers (eg Tape and Hard disk) remain the preferred option. Hard disks should be implemented within an integrated system.