I use FreeFileSync to mirror files from two different drives to another. Works great for me. Good simple application, easy to use. It does have a compare function to determine which files are new/changed.
Robocopy is my favorite. It can do it all and is already installed if you're using Win7.
+1 It is a very powerful tool. The following command will verify whether the files in the source folder will match the destination folder:
robocopy source destination /mir /l
The /mir switch makes a mirror copy of the source directory in the destination directory and deletes any files that don't match the source. Using the /l switch makes it do a test run only and doesn't write or delete any files. You'll need to execute it at the command prompt. At the end it will display a summary showing how many files will be copies/modified changed. If it is zero then you have an exact copy. If not, drop the /l switch and it will add the missing files.