Where is the missing pinwheel in skyward sword?

Answer 1

You could always ask the Sheikah Stone by the sparring hall in Skyloft, but to get the pinwheel go to the temple on Eldin Vocano and as you are facing the entrance go left a ways and it will be down a large sandy hill next to a guard tower. If you get lost start dowsing, you'll get there.

Answer 2

It's below the clouds, in Eldin.

You can dowse for it after talking to the man near the windmill it's missing from, and after having fixed Scrapper. Fly down to Eldin Temple, and head left. Turn left, and head down the slope - it should be at the bottom, near the fallen tower. Scrapper will come down, and carry it up in to the sky. Fly back up, and return it to the windmill!