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I often can hear that an external HDD has spun down, and figure it would be safe to unplug since it won't be writing any data.

Am I wrong in this assumption?

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Your best bet is to always eject (or unmount) the drive.

Basically, failure to unmount before disconnecting the device can result in loss of data and/or a corrupted file system.

If the external hard drive has in fact spun down, then unplugging it (accidentally or otherwise) is less risky than it would be if it hadn't spun down, but this won't remove the risk entirely. For example, in your question you refer to the fact that often you can hear that an external HDD has spun down, but this is prone to errors in judgment.

Also, not all external drives are the same. Some are USB powered, some have their own dedicated power source, some don't even use USB. What I'm getting at here, is that the only rule that can safely be applied to all external drives is to eject/unmount them first prior to unplugging them!

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  • Thanks! I was asking mostly because nearly always when I want to eject a drive I've not used for a while it can take 20-30+ seconds while it does who knows what. – Mint Dec 13 '17 at 0:35
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    Re the while it does who knows what, the way it's intended to work is that macOS (be it via the GUI or through Terminal) won't eject/unmount a drive until all data is finished being written to the drive. This I think most people do understand, but what also happens is that macOS ensures that no other programs are still using the drive. I guess it's easy for all of us to sit there impatiently (I know I do it too at times), but that extra 20+ seconds could be the difference between being able to access your data again or not. – Monomeeth Dec 13 '17 at 1:00
  • Yeah I agree, I just wish it made it a tab more clear what was holding things up. I know you can find out in the command line, it's just not super easy. – Mint Dec 13 '17 at 1:27
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Yes, it is. After ejecting the disk I would recommend turning off the power switch if it has one. If not, go ahead and unplug it.

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