David had to buy a new computer. He put dropbox on it, but now it's "barking" at him to buy more space. So he deleted some files, but dropbox also deleted them from his computer! What? Leo says that Dropbox really isn't a backup solution. It's a file syncing solution. That's why you only get 2GB for free. The reason why they delete it from the computer is that DropBox works through Sync, so it can match the files you have on dropbox and the computer. You can go into the Dropbox settings and turn off sync for folders you won't want so that it won't delete them. And if you realize it's deleted something, you have 30 days to undelete them and get them back. Also, look at the backups tab and smart sync tab to ensure it's not syncing anything you won't want to lose.
But it's also important to have a 3-2-1 backup strategy. Three versions, two different formats, one off-site. Or two off-site. Use Google Drive, for instance. You get 15GB free there.