You need very specific (and old) dvd readers to actually rip a game from a computer. There's only a select few out there, and you'll be stuck with purchasing one from something like Amazon or EBay. I suggest highly researching whatever drive you decide to buy as there are a lot out there that look like rawdump drives, but aren't. I personally have a LG GDR-8082n USB DVD Drive (External 8163b) that I used (once upon a time) to rip games, so I can personally vouch for that one. Either way, if you're that interested in dumping from a computer, you want to google "rawdump dvd drive."
I'll say this now, though -- rawdump drives can take HOURS to actually rip a game. DL games can take upwards to 4-6 hours depending on the mood of your computer.