Thanks for the recommendations guys.
davehg - I had thought that Silverkeeper suffered from the drawback I ineptly described in my original post. Let me have another go at explaining what may be a misapprehension on my part. I thought that a number of these programs use an updating process that involves replicating the entire database you are copying, in addition to any existing copy, and then deleting all the duplicate copies. Hence, if your back-up HDD is 250 GB, you can only back-up a maximum 125GB of data because the software needs to use the other 125GB in the backing-up process. Is this true of Silverlkeeper? (I'm guessing not as your back-up HDD is the same size as your original HDD.)