Asus T100 + HDD, battery impact?

Apparently, Croatia is one of the few countries where Asus T100 is available with the special keyboard dock that has a 500GB HDD.

Which is what I was waiting for. But now I remembered.. How much of an impact would a HDD have on a battery life, when used as a netbook obviously?

What I was able to find was the following:
- mobile HDDs are supposed to be using 0.5-2.5W (idle-load) (?)
- T100 uses 1.7-12W (load idle)
- T100 has a 31Wh battery
- T100 is measured to last 8.5-11h during normal use (depending on the source)

What am I extrapolating:
- system power usage during normal use is 2.8-3.6W.

What I fear, but don't know:
- if I take that HDD would be using 1.5.2W when used (say, I'm playing music stored on it), that could mean 50+% less battery life worst case!
Obviously I'm hoping that 1.8" drive or whatever would be in the dock actually uses much less power during regular usage (playing local music and videos, moving and copying some files, playing some older games like Baldurs Gate..).

I tried to find review from the same source of the same devices with/without HDD, but was unable to

Can someone help me?