Back up programs

I have 2+ TBs of images stored on a Drobo 5D. About 75% of these are made up of my Aperture Libraries which are now completely static. 6 months ago I decided to switch to LR5 and have processed all new images in LR. Monthly I back up these folders to a second EHD which is then stored at another location.
Over the past 5 days I have copied all my NEF images from Aperture into LR folders so I can access/work them from either program. However I now have a much larger monthly backup to the 2nd EHD.
Is there a program that will speed the transfer from Drobo to EHD2 by just updating what has been added/subtracted from the previous months work? (As opposed to just transferring the entire current LR collection.)

Several programs should do well for that task. Look for either a folder synchronization program (several in the App Store) or a backup program that supports incremental backups. I believe the paid versions of Carbon Copy Cloner and Super Duper do this.

Our Unix geeky contingent can probably suggest a command line alternative as well.

Thanks.
Will Super Duper work with two non Mac hard drives?
I use SD to back up my iMac HD but I am not sure it will work Drobo to WD EHD.

I hadn't thought about that until you asked the question. I wasn't sure so I did a bit of research. Here's one instance of someone backing up a Drobo to an external hard drive using SuperDuper. I don't think it lists the specific steps but apparently it can be done. I haven't verified it yet but I think Carbon Copy Cloner would work as well.

I am going to try "Sync Folders" from the Apple App store and see if that works. It is a pretty basic task but just a large folder.