First it is best to use Discjuggler to burn DC CDI images. Now you have to understand that the Dreamcast reads GD-ROM's at "1.2 GB's". GD-ROM's were made especially for Dreamcast. You can not buy blank GD-ROM's to burn on !! Dreamcast's Can Not read CD-RW or DVD-R's !!
You can only use CD-R's. The main problem is CD-R's can only hold 700MB . So if you have a 800MB DC ISO then you can't burn it !!
The next problem is there are a lot of bad DC ISO's floating around that won't Boot !! Because they weren't ripped properly !! If you come across to one of these bad DC ISO's when you try to run it all you'll get is a music screen on your DC or DC emulator.
So you have to look for DC ISO's in CDI format because they are self-boot.