Here is a summary of all the reasons to back up your data.
Back Up Your Data Now or Cry - James Tanner
It is about time to get back to this subject. One of my last blog posts talked about the need for your own desktop genealogy program. See Update of Do you still need a desktop (local) genealogy program? In focusing on this topic, I neglected to emphasize the need to back up all of your genealogical information. Since I am currently serving as a volunteer/missionary at the Maryland State Archives digitizing records, I can daily see examples of documents that are almost lost due to water damage, mold, and other causes. This daily experience has made me even more hyper about having more than one copy in more than one venue of everything I want to preserve.
Watch the video. Think about what might and will happen to all your precious documents and genealogical research. Then get busy and back up everything.