When adding a harddrive to the server storage, there should be a warning to confirm are you sure you wish to add drive, because if a user had important data on the drive they are adding, they will lose it.
If the end user reads the documentation this would not occur. Also, there is some discussion of adding a feature which allows for the migration of an existing drive with data into WHS. Here is a thread to read and one post contains a link to the Connect suggestion: