If you take lots of screenies, there's one thing to consider: if you download and install the GW client on the new drive, and then replace the .dat file and template folder, but put the screenshots in a NEW folder, you will start over on your screenshot count.
Why does this matter? When you reach a certain point, you can only take solid black screenshots. I have no idea why, but reinstalling GW is the only thing that lets you take new ones again; simply renaming the old folder once it has started doing that won't do it, even though the numbers on the file names wil start over!