Near-line, fear-line, which way to go?

We are setting up a Drobo for a local non-profit. I'm new to using Drobo. I'm planning on using Super Duper for my backup program. I am about to back up 4, 500 GB external HDs to a new DROBO unit. The DROBO has 4, 1 TB drives, which give me about 2.71 GB of space, given the way DROBO works.

This backup will serve as near-line storage for our client. The files on the drives are all photos, Motion, QTs , Compressor Exports & FCP projects and all associated render files, backups etc. Some of the files on a particular drive are the same, or slightly different from the exact same files on another drive. For production reasons, we had to duplicate source files. But for this backup I want to copy EVERYTHING, even if its redundant.

I have to back up the HDs one drive at a time, which means SD will have to be run 4 different times. I've read the SD guide, but am not sure which option I should use to backup all these files.

If I use Backup- all files, it seems it will delete the first batch of files I copy to DROBO. If I use copy newer or different, it seems it will NOT copy the duplicate files from a drive that I want included.

Any advise about the options/settings I should use to do this complete backup of all the 4 HDs.

Thanks!