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Maxtor has released a line of serial ATA Maxline drives it claims are 60 percent more reliable than desktop SATA drives and come with a five-year as opposed to three-year warranty.

Maxtor's Diamond Max desktop SATA drives have a mean time to failure (MTTF) of 600,000 hours, which is not good enough for heavy enterprise use. In comparison, enterprise drives such as Maxtor's SCSI Atlas have a MTTF of 1.4 million hours, but they come with a heftier price tag.

But recently a new and secondary form of disk storage has come into being, called nearline storage. It is for storing less frequently-accessed data which still needs to be online rather than held in a tape vault. SCSI drives are quite expensive for this and enhanced reliability SATA drives, such as this Maxtor Maxline series, are being used instead. Maxline has a 1.0 million hour MTTF rating, a low duty cycle and five-year warranty.

SATA drives tend to have higher capacities than SCSI drives. Like the Diamond Max, Maxline capacity ranges up to 300GB. Typically SCSI drives spin faster and hold less. Maxtor's Atlas 15K spins at 15,000rpm and holds 73.5GB. It costs around £325 per drive in the UK. A 300GB Maxline costs £135 and a 300GB Diamond Max £107. A SCSI drive nearline array could cost vastly more than an enhanced reliability SATA nearline array.

Plasmon has just introduced a RAID array using enhanced reliability SATA drives. The Raidtec FS3102 product offers 4.7TB of storage, a Fibre Channel host interface, and "professional quality SATA drives".

Paul Devine, sales director for Plasmon's Raidtec operation, said that "with cost-effective SATA disk drives, Plasmon has put SAN technology well within the reach of departmental applications and smaller organisations." In other words it is for secondary storage and lower duty cycle needs than enterprise online drives.

Unless customers know precisely which SATA drives are being used and what their reliability figures are they have no means of knowing how reliable their nearline storage is going to be. Manufacturers can be coy about giving out reliability figures. Maxtor's site for example provides them for Maxline drives but not its Diamond Max line. This makes life harder for storage buyers.

Array manufacturers under cost-pressure could populate their products with cheaper desktop SATA drives, resulting in customers experiencing higher failure rates.

There is no industry-standard measure of what duty cycle level and what MTTF rating is acceptable for nearline storage. Customers are advised to buy SATA nearline storage arrays with circumspection and choose arrays using drives of known reliability ratings.