Teardowns of Apple's freshly updated Time Capsule have led to questions over what constitutes a "server grade hard disk" as per the company's promotional material, given it appears a regular Western Digital drive has been used. According to Apple, the new Time Capsule offers either 2TB or 3TB of "server grade" storage; when Hardmac opened their 2TB unit up, however, they discovered a standard WD Caviar Green HDD inside, rather than a specific enterprise-spec drive.
The exact definition for what comprises "server grade" isn't clear, though it's generally acknowledged to include a MTBF (mean time between failures) in excess of 1m hours. Unfortunately, Western Digital doesn't quote MTBF figures for its regular consumer desktop drives, into which category the WD20EARS HDD Apple has used falls, and the company's press team couldn't give us a figure. What they did say was that there are specific feature sets for the server market that enterprise-class drives are geared for, and that the requirements needed between desktop- and enterprise-class drivers are definitely different.
Where that leaves the Apple Time Capsule is uncertain. This isn't a new issue: the original Time Capsule claimed "server grade" drives but used models from Hitachi's Deskstar range. At the time, Apple said that the HDDs were the same 7,200rpm models as used in the Xserve servers, with a higher (but unspecified) MTBF.
Any single-drive solution is always going to be less safe than a RAID array with multi-drive redundancy, so a Time Capsule shouldn't be the be-all and end-all of your backup strategy. Still, we'd like to see more accurate figures for how long the drive inside is expected to last; 3TB certainly implies that buyers will be using it for some time.