We make a product containing, among other things, an LCD and a hard drive. We have until this moment been characterizing the product lifespan based on the LCD backlight's rated lifespan, since if you look at the speced MTBF column in our BOM, the CCFL has the smallest number.
Recently, however, I was asked to add spindown-HDD-after-idle functionality to reduce acoustic noise. Then someone said "it should also increase drive lifespan greatly" and I started to research this topic using the full 200-page drive datasheets instead of the one-page spec sheets we used previously (which is where we got the single "MTBF" number). Now, I'm thoroughly confused. I'm trying to work out some reasonable defaults to reduce drive spinning time (= reduce noisy time) without overstressing some part of the mechanism and inducing premature failure.
Looking at one particular drive - IBM DJSA-220 - the drive is rated for 5 years or 20,000 power-on hours, whichever comes first. At 24/7 power-on, that's about 2.28 years. However, the assumptions in that lifespan are: less than 333 power-on hours per month (not valid for our product, which is normally powered up 24/7), and seek/read/write operations occupy less than 20% of power-on hours (might or might not be valid for our product, depending on exactly what the user is doing). The datasheet essentially says that all bets are off if those limits are exceeded. Furthermore, the drive is rated for 300,000 normal head unloads, and 20,000 emergency unloads. Our product's power switch is an emergency unload. Me spinning down the drive in software is a normal unload.
These questions become even more interesting for some other drives we use regularly, e.g. Fujitsu MHK2120AT. They are rated for the same 5 years/20k hours (250 hours/month maximum, and 1 power-cycle per day REQUIRED, but no more than 50 spinup/down operations per day!), but they are only rated for 50,000 spindle start/stop operations total (it appears this limit is related to the associated head load/unload operations, not specifically the spindle motor). If the user has the drive auto-spinning down every 15 minutes (not unreasonable, this means less than 1.5 years before something dies. Weirdly enough, the
250 hours/month and max 50 spinups per day limits are removed if you can guarantee to keep the disk envelope at 48 Celsius or below (which we can't).Is there anyone else who uses 2.5" IDE hard disks in an embedded system, and has developed a sane method of choosing default power management settings? I would like to be able to say "yes, guarantee it for three years", although two would do. As a secondary point, I'd like to know what to put in the product's instruction manual, since the HDD sleep time is user-configurable. Do I say "anything other than our carefully tuned default setting will reduce the lifespan of your hard disk"? Or should we just put a waiver in the warranty saying the HDD is only warranted for 12 months, and after that time only the labor is free?