Hi studybuddy,
You ask a good question - and there are a couple of potential ways to deal with this issue. First, you could use your erase board/marker and take a picture of your notes before you wipe the pad down (so that you could review how you worked through each question when you review your CAT later on). Second, you could cut some paper pages (for example, from a legal pad) to fit the size of your white board and use the marker that you have for the white board to take notes on those paper pages. That way, you'd be able to keep your notes and reasonably simulate how you'll end up taking notes on the GMAT.
How have you been scoring on your recent CATs/mocks?
GMAT assassins aren't born, they're made,
Rich