Should I spin down Mac Mini's drive? I have a Mac Mini (2010) that I use as a home server with a DAS (2 drives) attached over USB. Everything is on 24/7.

From all my research online, it seems that spinning hard drives up and down constantly is actually only worse for the drive so I've unchecked "Put hard disks to sleep when possible". Now the internal drive as well as the two externals are spinning 24/7.

The other reason I keep the drives spinning is that if I do have the drives on auto-sleep, I keep getting I/O Errors from the kernel log for my two external drives on USB which was very worrying. Keeping the drives spinning makes those errors go away entirely.

The problem is that the internal drive on the Mini crashed recently and it was only about 7 months old :( Thankfully it was under warranty but the next time I probably won't be so lucky.

This has me doubting my always-spin approach. All the stuff I read previously was for 3.5" server drives, maybe the 2.5" drives are different in this regard and I should sleep them when possible? Or was I simply just unlucky with that crashed drive?

Additionally: Should I want to sleep the 2.5" internal drive, is there a way to sleep the internal but keep the externals spinning? Seems like the pref panel as well as pmset apply the setting to all drives.

Thanks!
Ideally, you should set a longer spin-down time so that it is not occuring constantly. A time to spin down of after being idle for 4-6 hours would be better. This gives you the benefit of saving power for long periods when little is being done on the drives, e.g. during the night or while you are away from home at work, etc. Having longer times also allows for less slow down when you are using your drive since you will have a reasonable time-frame to get files and work done before the drive trys to spins down on you. 

Use pmset from the terminal to set up a custom interval that works best for you.

Others our claiming that spin down or not is really not that big of a deal in a home / SMB environment.

As for the differences between the 3.5" and 2.5" drives its safe to  assume they are the same or similar. Even a comparison between what you said was a 3.5" server drives is not the same as a comparison between a 3.5" consumer desktop drive, they are not on equal ground just because of form factor. It really comes down to the manufacture quality and is really one of those you get what you pay for kind of situations and OEM drives even in Macs are still a lowest bidder kind or process. Here is a discussion on the reliability of 2.5" vs. 3.5" drives.

As for the differences in trying to set internal or external drives independently it looks like its not possible since the setting is really only intended for the startup disk according to The X Lab -- Hard Disk Sleep 


  Another undocumented aspect of Put the hard disk(s) to sleep when possible is that this option generally applies only to the startup disk. Non-startup hard disk drives — external FireWire® or USB® drives or additional drives installed in a Mac desktop — may ignore this setting.