Yeah, I just use a marker too. It's only a backup and you already have the artwork on the original DVD you bought.
I don't use Sharpie brand though ... the strong petroleum based smell always made me wonder if there might be a long term chemical reaction with the plastic to worry about (I don't really know). And the sharpness of the point always dulls quickly too. I use Staedtler Lumicolor Permanent markers in Medium, Fine, and Sharp points (depending on what info and how much info I write onto the backup disk). They are available at most art supply stores or at least they can order them.
If you just gotta have the artwork on your backups, printable media (costs more) and epson printers with the disk printing feature is the way to go. But what I am unsure about is that, when you inevitabley have to clean a dirty disk with alcohol in a little sprayer bottle, whether any alcohol that accidentally gets onto the printed side (via overspray, your fingers or just a drip-over) will damage the printed artwork. ... or whether using water/dishwashing soap to clean a printed disk will damage the artwork.
Whisperer