copy folders to ext drive with new data each month

[Buddy123]

HI , I am hoping that someone can help me , I copy
"My Documents folder" once a month but have had to use multiple ext drives and delete an old copy first before I can add a new one, as it comes up saying already copied... is there any simple way around this .. I do this as I am using two computers and my main computer is where I do most of my psp and stuff , so like to keep my newer pc up to date . Hope this all makes sense that the question is allowed.