Yes, they are empty. They're new out of the box.
Technical support just e-mailed me that the acceptable disc formats are:
CD-R/RW, DVD+R/RW, DVD-R/RW, DVD-R DL, DVD+R DL, DVD-RAM, BD-R/RE, BD-R/RE DL
I am using a CD-R, so I guess I have to use a CD-R/RW instead.