Hard Disks - cheap way to store backups

Disks have become a very attractive backup target these days. Here is a quick summary of some of the reasons why is it so:

• Cost: The biggest reason that disk has become such an attractive backup target is that the cost of disk has been dramatically reduced in the last few years. The cost of a reasonably priced disk array is now approximately the same price as a similarly sized tape library filled with media. When you consider some of the things you can do with disk, such as eliminating full backups and redundant files, disk becomes even less expensive.
• Reliability: Unlike tapes, disks are closed systems that aren't susceptible to outside contaminants. In addition, the actual media of a hard drive is, well, hard when compared to a piece of tape media. The result is that an individual disk drive is inherently more reliable than a tape drive. Disk drives become even more reliable when you put them in a RAID array.
• Flexibility: Generally speaking, tape drives can only go two speeds: stop & very fast. Yes, some tape drives support variable speeds. However, they can usually only slow down to about 40% of the rated speed of the drive. Disk drives, on the other hand, work at whatever speed you need them to go. If you need to go a few hundred megabytes per second, put a few drives in a RAID group, and blast away. Then if you need that some RAID group to write at 10KB/s, go ahead. Unlike tape drives, disk drives have no problem writing slowly, then quickly, then slowly, then.... You get the picture. This makes disk a perfect match for unpredictable backup streams. Once all that random data has been written in a serial fashion on your disk device, the disks can easily stream backup data to tape - if that's what you want to do. Some people are foregoing that step altogether and replacing it with replication. Try doing that with a tape drive.
  

Disk based backups are also an extremely economical way to bring completely automated backups to small & medium businesses (SMBs). While a large tape library can be very inexpensive (on a dollars-per-GB basis) and very expandable, the same is not always true of smaller libraries aimed at the SMB market. The big challenge is expandability. The less expensive a tape library is, the less expandable it usually is.