How much power does a hard drive use?

This is not as straight forward as it sounds.

Specs from Western Digital's site for a WD 3TB Green Drive:

• Read/Write 6.00 Watts
• Idle 5.50 Watts

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 four 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 four drives?

Most of you are way off here. You are confusing peak and average draw. 1.78 A is the start-up current value.

When calculating power dissipation, both 5V and 12V are considered. All the power to a drive is converted to heat, with 99% being dissipated by the drive and a small amount dissipated over the interface. Friction in the bearings and airflow on the platters results in heat. Losses in the drive motor electronics and windings and by the chipset waste the rest.

A 6W drive through an 80% efficient power supply will draw about 7.5W at the wall.

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

Update:

Lots of hate in the comments. I encourage commenters to read exactly what this answer says and don't read anything else into it. This answer makes exactly two claims:

1.) The OP's calculations were correct

2.) The OP didn't understand the term "power dissipation".

The answer does not claim that WD specs are correct or that they make sense. In fact I would guess that WD has used the term "power dissipation" incorrectly (as others have already suggested) and should have instead used "average power consumption", but that is just a guess.

Some commenters have mentioned that all energy is lost to heat. This is mostly true, but is technically incorrect because some energy is lost to magnetic and gravitational forces. Since the primary loss of energy is most definitely in the form of heat it does make us raise an eyebrow to WD's supplied specs which do not include any peak or average power consumption numbers (a fact that one commenter for some reason blamed on this answer) and may suggest that the HD in question breaks the law of energy conservation (a fact that another commenter for some reason blamed on this answer).

So again, this answer does two things. It confirms that 12V x 1.78A = 21.36W and it provides a definition for the term power dissipation. That's it!