First, you are to be commended for wanting to do a backup. Far too many people get along dumb, fat, and happy until their HDD fails then come a crying, wanting to know how to get back the lost data (which it rarely can and can be horribly expensive trying to).
I clone my data drives using free Macrium Reflect and image my system (boot) drive. Cloning is more reliable than just copying files but the entire disk or partition has to be cloned each time; you can't just add a few files at a time. Imaging is also more reliable and also more flexible, especially for system files, and can be added to (a paid version of Macrium Reflect would be required to add to an image). The downside to an image is you can't easily access a single file or folder like you can from a clone.
As firebird pointed out, it's going to take a certain amount of time for a disk to be read or written to and there isn't much you can do about that. 700GB is just going to take a while. When I'm running a backup of a data drive (over 900GB; it takes two to three hours using internal SATA connections; USB would be much slower), I start it just before going to bed and just let the computer do its thing. It's usually done by the first time I have to get up for one of my nightly pit stops (old age is the pits) and I'll start the second backup before crawling back in bed (I like to have two local backups on hand).