I am looking for Mac software to synchronize two exFAT formatted external hard drives, for backup. Hard drive B should reflect all changes from hard drive A (including deletions) but hard drive A should not be changed.
I'd recommend either:
Both are great for cloning and synchronizing drives or even part of drives. I've used both over the years.
At the moment, CCC offers a 30-day free trial with all features.
Super Duper will clone drives for free, but to only update changed files, you have to buy it.
In both cases, I think you should be able to evaluate each for free and decide which one you like before buying.
time machine (built-in OS X backup app) with some terminal tweaks: http://hints.macworld.com/article.php?story=20140415132734925
Note that it involves creating an OS X specific partition on the second drive. Actually, since it's just a backup drive the exFAT requirement does not make any sense there anyway. You don't want to back up from OS X and then back up from Windows as well from the same location to the same location.
Just use rsync. This should work particularly well since you're only doing a one-way sync.
rsync -av --delete /Volumes/<disk1>/ /Volumes/<disk2>/
Will sync disk2 to the contents of disk1, recursively (-a), with verbose descriptions (-v) and deletions (--delete).