Thanks for the replies -- good ideas! I also just came up with another method myself.
In the Media Library, select Audio so that all songs in your library display. Next, horizontal scroll over to the Date Imported column. Click the header on this column until it is organized from most recently imported to oldest imported. Now, all you have to do is write down the last time you backed-up all your files. To update, just left-click the most recently imported file, vertical scroll down to the date in Date Imported column that is just after your last back-up, hold down the Shift key and left-click that file. Now, all the files you need to back-up are highlighted. Simply drag & drop those files into the CD burner on the tree. That is it. Only downside is that MC 9 does not go back past 80 days for Date Imported, so if you wait more than 80 days, it may generate confusion. Wish the programmers would fix this problem.