There are many options.
You could buy a commercial product such as one by Acronis. This will probably be the simplest as the expense of a few dollars. Just create a scheduled backup task that backs up one drive onto the other drive. You could use drive images or just clone one drive to the other.
You could find a free backup program and try that. I have no experience with these so I can't recommend one, but here is a simple list of free backup programs to get you started.
Or, you can write your own and run it as a scheduled task. Assuming you're using Windows, you could write a batch file as simple as:
xcopy X:\ Y:\ /e /i /y
robocopy X:\ Y:\ /COPYALL /MIR
which will copy everything from the X: drive onto the Y: drive (assuming you have the drives mapped as X: and Y:, of course). I'd recommend the
robocopy option (you'll need to install it using the Windows Server 2003 Resource Kit Tools if you're not on Vista or later) since it has more options available to you if you want to customize how it works, and it behaves better copying over the network.