I find when I have both Terragen 3 open and have a memory card in one of my slots, and then I remove that memory card (without ejecting), Terragen will continue to prompt me for that card until I reinsert it. Not sure why it's referencing the card in the first place (from my digital camera, not inserting anything from the card into Terragen), but I thought i'd let the programmers know anyway. It's only a small annoyance.
Oh yeah, I'm using Windows 7.