It comes from experience. Experience tells me that about 50% of some manufacturers drives last 4 years. (specifically, WD and Fujitsu). With a standard deviation of 1 year, that means that around 15% fail under warrantee, at between 2 and 3 years. So you can get lucky and have a good WD drive last 5 years (~15% chance), 6 years (~3% chance), or longer.
Seagate makes several qulity levels of drives, their high end models tend to last around 5 years.
Quantum drives are hit or miss.
Maxtor drives are hit or miss, but generally tend to last around 5 years
IBM drives tend to last around 7 years or more, but certain models (such as the 70GXP) had problems.
the 3-year figure comes from their warrantee period.
SCSI drives last longer. It's hard to say why, except that maybe they use better bearings in the motors and heavier duty discrete componenets/heavier traces on the PCB's. They have to last longer, because they're warranteed longer.
WD seems to have a real quality problem.
What's the frequency, Kenneth?