Until recently, hard disks weren't practical for backup, mainly because the necessary drives and RAID controllers were pricey. But D2D storage is affordable because RAID configurations are now being built on the inexpensive, high capacity ATA/IDE technology. Silicon for an ATA RAID controller today, for instance, costs less than silicon for SCSI (see costs chart, below).
Reel Hard Choices
There are advantages and trade-offs to both disk and tape technologies. Tape remains the dominant technology for long-term storage of archival data because of its ample capacity, portability and life expectancy.
With hard disk drives, capacity is fixed, so if you want to double your RAID array's capacity, for instance, you have to buy another RAID enclosure with an equivalent number of drives or buy higher-capacity drives. A tape cartridge, on the other hand, instantly and inexpensively adds capacity: With each tape you buy, you can double your tape-drive capacity at a fraction of the drive cost.
Tape's biggest limitation, of course, is its large backup window. Tape is sluggish because of its linear nature and slow transfer rates. It takes longer, for instance, to access data in the middle or at the end of the tape. But if speed is less of a concern than capacity for your organization, go with tape. Be aware, though, that the more backup tapes you fill, the more you'll have to manage.