Last time I logged in as admin via ssh shell, actually today, I saw that there was a "new" function I have not seen before (but it still could have been there for a while.) This was something like "backup to usb".
Some time ago when it was not a standard function I actually made such a modification/function myself, and that worked very well. I guess the built in sme function will also work well.
Maybe it could be an idea to just install the extra hd in a usb hd drive box and just to use the standard bacup to usb (hd) function ? (As mentioned, I have never used the buildt in bacup to usb (hd) function, but I guess it will work.)
Actually these days, an 8 Gb memory stick could also do it ? (I also don't know if this will work, but I would believe it.)