no, seriously - since you have ongoing problem with HD, there is a good chance that you won't have time to wait for DVD to be burned. I like the suggestion with using ext hd. it's cheap and fast and! reliable. Ideas with dar/tar/rar etc are all good, but you do want to check that whatever you burned on DVD got there correctly and burned correctly. I'm no saying it's can't be done, but the question is time and effort.
Keep us posted , i'm curios what will be your decision and outcome.
Also, im not entirely sure about your comment "sure that there a big big problem once we need to backup such amount of dvd because there is no solution up today under linux". I think there should be a box full of DVD burners that you can attache to your system. At least i remember a few (`10 ) years ago such monstrocity existed for SUn/Widows when DVD were considered "limitless"
get 2 drives, raid them and be done. Seems like you are looking for the production solution. no ?