Not all zip drives are the same. I have one of the first ones that came out in 2001, I still use it with no problems. I do think you should have a few different places that things are backed up too. When it comes to zip drives and designs, some require more unused space to run successfully than others, plus some don't like being put on a large zip, I've heard of 2 to 4g being the largest they like but I've used way larger ones on mine. Alot depends on your computer too.
Rennie: it depends on how you "burned" your cd, you can chose to burn them where you can add to it like a zip drive or burn them and the cd is closed where nothing can be added or deleted from it.