iTunes says the CD isn't blank, but it is...

I just burned a playlist to disc, then when I attempted to burn a second copy, I got an error message, saying something about the burn being aborted and suggesting that calibrating discs before burning might help. It asked "Do you want to calibrate discs before burning?" Trusting Apple's advice, I said yes. When I then tried to burn the playlist, it said to insert a blank disc. My Computer showed nothing on the disc, but I grabbed another blank, and another, and another. Each time, it said "Checking media...." for a few seconds, then nothing.