How to know if laptop can read blank disk?

Ok, I had bought a 4.7gb dvd+r in hopes of burning a windows 7 iso image, but when i popped it in, my dvd rw drive cannot detect it. It can read other dvd movies fine. Should I buy another dvd? this time dvd-r? I have read in this post [ http://www.askdavetaylor.com/whats_the_difference_between_dvdr_dvdr_dvdrw_dvdrw_and_dvdram.html ] that i kind of should. I want to make sure. Please shed me a light on this problem.