I've burnt several backups at this point. I've used verbatim, HP, and memorex discs and the two coasters I've burnt were entirely my fault. I haven't had a single disc that didn't work 100% when I applied the correct settings in imgburn.

I wonder if it's because of the quality of my burner. I bought it a year ago and paid $100 for it. It's a plextor PX-716SA and I've never burnt a coaster with it, unless I did something wrong. Does anyone have any experience regarding this?

The burner does play a part in reliability but there is so much more to it than that and to be quite honest the Plextor DVD Burners are horribly overrated. They are good drives but their pricetags rarely justify the results when you stack them up to their competitors. Other factors include: Your individual 360 DVD-ROM/firmware revision, your particular DVD Burner/firmware combo and the media itself. Read the first link in my sig for more info.