Re "server grade" -- most hard drives available at retail and sold in consumer products are rated for only, say, 100k power-on hours, and that only if they're active less than 300 hours per month. Drives sold for use in disk arrays, OTOH, have a MTBF of perhaps 1,000,000 hours and are rated for continuous usage.

For example, the HTC426040G9AT00 in my laptop has the following specifications:

• less than 333 power on hours/month
• operating (read/write commands) less than 20% of POH
• Continuous motor spinning should be limited to 48 hours period
• they don't actually quote the MTBF

By comparison, here's the similar spec list for a HUS103030FLF210 fibre channel drive designed for enterprise use:

• MTBF: 1,200,000 hours
• 5 year lifespan ("Life of the drive does not change in the case that the drive is used intermittently.")

Note that they don't even mention POH or operating duty cycle; in enterprise applications, these are assumed to be 100%.