SyncBack (freeware) is an application that I have been using for several years without any problems whatsoever and from what I read in various forums, so to do others. It can be had from here:
http://www.2brightsparks.com/downloads.html#freeware
One word of caution though, when setting it up (and it's very easy) ensure that you read through and understand the various options
very very carefully - I can't stress this enough. For example, as the name of the application implies it can be set up to 'sync' one drive to another. In simple English then, following syncing two drives will be exact mirror copies of each other. However, it can also be set up to make backups i.e. a folder or folders on Drive C onto an external hard drive (EHD) - drive E say, and this is sometimes where difficulties can arise if not fully understood.
I have mine to backup data files on Drive D to an EHD with the option to delete data from the EHD that it no longer finds on the original drive D. This means that if I start out with X amounts of data on Drive D and decide to delete Y number of files I now end up with X minus Y number of files on Drive C. When running SyncBack, it will check the backup files on the EHD and if it finds files on there from a previous backup that are no longer on the original (Drive D) these will be deleted. There is also an option of having nothing deleted from the backup copy but of course the two drives are no longer mirrored.
The practise is a lot easier than the theory believe you me and I am attempting simply to make the point of double checking your set up of the application prior to running it so that you know exactly what is going to happen. The application helps in this direction by presenting you with red font if there is a risk of something not being quite right and asking you to recheck.