No. "Only for that purpose" means that you never use it. This is trivially detected: the OS event logs will report that it hasn't been booted in 6 months, and no files have changed in that time. It's obvious that you haven't been using that system in the time you were out of the country; why did you have it with you?

For this to work, you need to actually use the dummy partition for non-sensitive matters -- and be absolutely religious about separating the sensitive tasks from non-sensitive, which is much more difficult than it first sounds.

I actually carry a lighter computer when travelling than the one I use most often. So yes, it may have gone months unused and it may have just a few tools on it, and little to no content.

There is nothing unusual about that.

I'd feel ok telling the US borders I've got a special laptop just for them.