External HD Backups

How do I explain this...

I have two HDs. Hard drive A ("HD-A") is my master. Hard drive B ("HD-B") is my backup.

On Jan 1, I took my Premiere Project (files, sequences, raw footage, tons of GB's of material) and copied it from HD-A to HD-B. I now had a full backup of HD-A. Yay!

Later, after making several revisions to my editing project, I'm ready to backup once again. But when I bring the contents of HD-A into HD-B, I don't want to 'MERGE' the folders (because this creates duplicates, which become cumbersome and waste space), nor do I want to "REPLACE", because replacing sounds permanent, which is scary.

Instead, I would simply like to 'update' HD-B, by adding only the new files/edits/exports I've made since my last backup.

Is this making sense? How do I do this?