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I have an SSD in my MacBook Pro as my boot drive. It also houses all of my applications, documents, etc. I replaced the optical drive with a mount for the original hard drive, which I only occasionally need to dip into.

Would I gain any benefits from ejecting the original hard drive when I'm not using it? I immediately noticed that my machine is quieter - would there by a noticeable impact on battery life?

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How much is occasionally? –  imtheman Sep 11 '12 at 23:13
    
Once a week perhaps. –  Ben Packard Sep 11 '12 at 23:15
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3 Answers

up vote 5 down vote accepted

Yes, removing the original drive when not is use should aid in preserving your battery. The less things the battery has to power (especially things with motors, like optical drives, and traditional hard drives), the better.

As you said, the machine is quieter without the drive. The noise is likely either the drive spinning (ie. using power), or a fan in the machine spinning (ie. using power) to provide additional cooling.

If you really want to know how much of a difference. run the machine from full battery to low with the drive, then without. Try to do similar things during the test so the test is fair (ie. don't web surf on one test, then transcode video for the other). I would imagine you would see a difference in the times.

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Just to be clear, I am dismounting it rather than physically removing it. –  Ben Packard Sep 11 '12 at 23:29
    
Ejecting may still help though. If ejected, the OS won't be running virus scans against it, file searches, etc. You should still see some benefit. –  dangowans Sep 12 '12 at 10:38
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Just dismounting will not give you any benefits as the hdd would still be spinning. You would need to spin the drive down to see any battery life improvement.

Or you can eject it using this script http://forums.macrumors.com/showpost.php?p=9596575&postcount=5

do shell script "touch /Volumes/YOURDISKHERE/.dummy;
diskutil eject YOURDISKHERE"

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This eject script does something different to right clicking and ejecting the drive? –  Ben Packard Sep 13 '12 at 11:24
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There would be an impact on battery life if you removed the extra HDD. If suspect it is noticeable, but the only way to really determine that is to measure it.

Time how long your battery lasts if you:

  1. Leave the drive in the optical bay.
  2. Physically remove the drive. (Time gained here is the maximum you can gain).
  3. Logically remove the drive (umount) and spin it down. It might do this on its own if it is idle for long enough periods. Some drives spin down on their own if left idle (regardless of OS command). This is set by the manufacturer and per model, but you can usually alter that. OSX might also have a setting for it (from the OS side), but I am not familiar with that OS.

Still, both options (remove and power down to standby) should stop the drive from spinning, thus using less power and creating less heat.

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