Backing up System Files? Deleted Files?

Funny! I just bought an extra h-d on the weekend. Got my docs & files all transferred over.  Do NOT want to use it as a routine back-up system … but would like to back up my computer just once – now – and then I’ll use the hd as the destination to save ALL files from here on in.  (So similar, really, to Allison’s post, which is why I’m replying to hers.  Strength in #s, right?)

How do I use the external hard-drive for backing up just once … while saving all of the files/docs that I’ve just transferred over to it?

Thanks.