I am assuming you use a PC compatible.
I use microsoft's sync toy which backs up to an external hd, there are various options you can set in my case I get it just to add only new files or update files that have changed. You can get it to remove files that are not present anymore in your main folder if you wish.
Originally I used to copy the whole lot but it was getting a tad large and taking over an hour to copy.
you can download it form
here there are both 64 bit and 32 bit versions available.
It is free as well
