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This is not as straight forward as it sounds.

Specs for a WD 3TB Green Drive:

Read/Write 6.00 Watts

Idle 5.50 Watts

Source: http://www.wdc.com/global/products/specs/?driveID=927&language=1

Looks fine right? Look at this part of the spec: "12 VDC" and "Read/Write 1.78 A".

It was a long time ago, but when I was in college that would mean the drive uses 21.36 Watts (12V x 1.78A).

21.36 Watts is a lot more than the claimed 6.00 Watts.

I want to put 4 of these in a RAID 10 array, so I want to know the actual max power requirement.

Thoughts? Is this a simple typo? Do I need to plan on ~85 Watts of power to support 4 drives?

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That "spec sheet" is a joke and incomplete. There's no mention of current draw for the +5 volt supply. I remember when 5.25" HDDs had full product manuals with graphs for all operations for both voltages. –  sawdust Mar 14 '13 at 8:41
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3 Answers

Your calculation is correct, but your understanding of the term power dissipation is lacking :)

Electrical Specifications
Current Requirements
    12 VDC
    Read/Write  1.78 A

Power Dissipation <-- Energy measured in watts lost as heat
    Read/Write  6.00 Watts
    Idle        5.50 Watts
    Standby     0.80 Watts
    Sleep       0.80 Watts
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Wow. Thanks! I had no idea a hard drive would use that much power. –  Jay Wen Mar 14 '13 at 4:06
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If 85W seems like a lot for the PSU you are planning to use for this system - do not forget that during start-up/spin-up the current drain could be almost twice as much (up to 3A per drive).

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Thanks. This is certainly an eye-opener. I haven't looked at a desktop/server hard-drive since ~2004. I knew I would be looking at more power than a laptop drive, but I didn't know it would be that much more. –  Jay Wen Mar 14 '13 at 7:37
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21.36 watts sounds about right. You can use a general rule of thumb for about ~25 watts per 7200 rpm drive.

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