adding to a CD?????
not a good idea
http://www.pcmag.com/article2/0,1759,25430,00. asp
Statistically HDDs are far less likely to go faulty than CD/DVDs so in reality it's probably safer to hold on to your pikkies until you have a good few Mb and then burn them to a new CD disc
If you look around you should find good quality BRANDED discs for 25p each (worth every penny if the data is important)
that way if a CD becomes unreadable ... you only loose a few ... not the whole lot
then from time to time copy your CDs to your HDD and burn a complete CD - then you are pretty bomb proof