Never rely on a single copy of data. Keep your source (primary, or raw) data separate from your active data, and always make a copy of it before working on the data. CD's and DVD's are not reliable as long-term storage options. Their life expectancy is only 2-5 years, and they need to be stored under the appropriate environmental conditions. Hard drives have a life expectancy of 4-6 years. USB (thumb) drives are not a good option. They are easily lost and stolen.
At the very least, you should have one separate backup of your data. Best practice is to follow the 3-2-1 Rule. You should have three copies of your data, in two different physical locations, on at least one different type of media.