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I'm not big on saving electricity at all and I'm trying to decide whether to select Mac's “Put hard disks to sleep when possible.”

What are the pros and cons of doing so?

For example, can I expect the hardware to last longer by keeping the hard disks on 24/7 versus putting them to sleep?

I'm using MacBook Pro (2017) running Sierra (10.12), but question is also interested in other (future) versions.

And why is “…hard disks…” plural when there's only one?

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Advantages:

  • on older traditional rotating disk drives it will save some power

  • on the same type of drive it often parks the heads making it less fragile in cases of unplanned moving/shock/drop

Disadvantages:

  • rotating hard disk drives will live shorter due to the fact that it’s the starting and stopping that kills them, not running idle

  • slower initial access to the disk as it starts after it was put in standby (it needs to spin up before it can be used)

None of this really matters to solid state disks as they are pure electronical devices with no mechanics. Their power use is directly proportional to disk activity. While they often do have extra low power modes, you won’t actually benefit much from it since it is not much less than its normal power regulation.

The reason this is plural is because it can also apply to external disks, and desktop macs used to have space for up to six harddisks. The 2009 Mac Pro has 4, for example.

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  • Do you mean that if I only use SSDs, I should leave the setting on.. or off? – Pacerier Jan 25 '18 at 18:21
  • If you only use SSDs it doesn’t matter what you set it to. I’d suggest leaving it off by default in case you connect external hard drives. – John Keates Jan 25 '18 at 21:53
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I'm not a 100% professional with hard drives, but here is what I would think some of the disadvantages and advantages are.

Advantages:

  1. Like you said, an advantage is the disk life might be extended. Though this extra amount of life might not be that noticeable.
  2. Battery life - The hard drive obviously uses electricity to power it. If the hard drive is put to sleep it will turn off, thus not using any electricity which will help the battery last a bit longer.

Disadvantages

  1. Longer wake time - When the computer goes to sleep the hard drive will completely stop, so when the time comes to wake the computer the hard drive will need to spin up. There will also be a delay whenever the hard drive needs to spin up after it's been put to sleep.

Hope this helps.

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    "Longer wake time" – If this is 'longer wake up time', how so? If it goes to sleep, everything is put to sleep? What difference is then there for the setting? – LаngLаngС Jan 22 '18 at 22:11
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You don't have to worry about that setting. Apple in their infinite wisdom have decided that it should have no effect. Whether you enable or disable it, it will put your disks to sleep. At least on High Sierra.

Why they've chosen to keep the setting around is anyone's guess. Maybe it's bug — these days that seems very likely ;)

EDIT: As for how I know this, it's based on me trying the setting out on High Sierra to prevent 10 attached USB drives from spinning down, and confirming that the setting does not have any effect.

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    Do you have a reference for any of what you are saying? – kojow7 Dec 12 '18 at 5:23
  • It might not have an effect on certain cases, but still kick in when needed. "10 attached USB drives" sound like a lot. The settings it not "NEVER put hard disks to sleep", but "Put hard disks to sleep when possible" e.g. whether to take advantage of more cases to put hard disks to sleep or not. – Hejazzman Nov 14 '19 at 8:34

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