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