French tech blog MacBidouille (aka Hardmac) tore down Apple's Time Capsule this morning, and found a hard drive inside that raises questions about Apple's claim that it uses a "server-grade" drive in its wireless router/data backup device.
The term "server-grade" is not an industry standard, but in a 2008 interview, Apple blog TidBits said Apple product manager Jai Chulani "clarified that the 'server-grade' drives in a Time Capsule are the same 7,200rpm drives used for Apple's Xserve servers, and that they have a higher mean time between failure (MTBF) rating than consumer drives."
MTBF is itself a questionable metric. There is no standards-setting body to advise on testing methodology or minimum ratings for enterprise-appropriate hard drives. A million hours between failures is sometimes considered the baseline, but as MTBF ratings are provided by the hard-drive manufacturers, you'd be right to question their reliability. Carnegie Mellon even found in a 2007 study on drive MTBF ratings that, as opposed to an industry-claimed failure rate of less than 1 percent, "in the field, annual disk replacement rates typically exceed 1 percent, with 2-4 percent common and up to 13 percent observed on some systems."
Even if we went fully accepted the validity of MTBF, we'd still have to question whether the 2TB Western Digital Caviar Green drive in the Time Capsule counts as "server-grade." A product PDF on Western Digital's Web site lists the MTBF for what it calls its Enterprise hard drives, but it has no such information for its consumer-oriented desktop drives like the Caviar Green.
A specification sheet for the Caviar Green (PDF) lists a statistic called "nonrecoverable read errors per bits read," and the Caviar Green drives all get a rating of less than 1 read error in 10 to the 14th power. That's the same error rate as Western Digital's Caviar Blue drives (PDF), which Apple uses in its consumer-oriented 21-inch iMac, and less than the WD RE4-GP, (PDF) essentially the Caviar Green enterprise equivalent, which has a read error rate of less than 1 in 10 to the 15th power.
We could consider other metrics to rate the Caviar Green's reliability. Its three-year warranty matches that of the Caviar Blue, but is less than the five-year warranty that comes with Western Digital's enterprise-class drives. We might also look at comparable drives from other vendors. The consumer-oriented Hitachi Deskstar 5K3000, for example, also has a 1 in 10 to the 14th power chance of a read error. (PDF)
Without a published MTBF rating for the Western Digital Caviar Green hard drive, we cannot say with certitude if the Time Capsule meets the decidedly loose standard for a server-class device. You could reasonably accuse Apple of taking advantage of our willingness to conflate "server" with "enterprise." You can make a server out of a $300 box of computer parts. An enterprise server, for any responsible enterprise at least, meets a higher standard, and we'd be surprised to find a reputable data center shopping for hard drives from Western Digital's consumer product section.
The most important question is all of this: should you still buy a Time Capsule if you believe Apple is overstating the reliability of its hard drive? If your livelihood depends on data security, we might ask what you're doing shopping for a $299 backup device to begin with. Otherwise, we still find the $299 2TB model offers a competitive price for its combination of storage capacity and wireless routing. We'll firm up our opinion with a full review later this week, including storage and networking performance test results.