Following yesterdays article on whether or not the drive used in the Time Capsule is server grade, comes a comment direct from Apple themselves.
According to TidBITS Apple has clarified that the term "server grade" is directly related to the MTBF:
Chulani clarified that the "server-grade" drives in a Time Capsule are the same 7200 rpm drives used for Apple's Xserve servers, and that they have a higher mean time between failure (MTBF) rating than consumer drives. The MTBF for server-grade drives is often 1 million hours (114 years), which is a measure of probability; in this case, that out of a set of drives with similar properties, an extremely high percentage will still be fully functional after several years.