You have 2 easy options. If the drives are the same size, you can do software RAID between them, so the 2nd disk is a copy of the first. Even easier is to set up the wonderful http://backuppc.sourceforge.net/
to backup whatever you tell it to backup. Of course, the backuppc solution can only run on a linux box, so if you only have 1, it is backing itself up, and fdoesn't help much in the even of a catastrophic failure, but if you just want the ability to restore yesterday's files over todays, it rocks. I deploy that all over.