I use a program called Acronis True Image - What it does is called incremental backups. I have it setup to backup only new or changed items (it checks both filesize and date stamps) every night at 2am... so, after the initial backup of the whole drive, it only does what is needed and I'm ensured my drives are synch'd every night. Every few months, I do format the drive and do a full backup again - this is just to merge all those individual incremental files down into one.. not necessary but is easier to work with.

There are a bunch of programs out there.. but I like Acronis the best.