How to synchronize contents of two external drives? I have two external hard drives (ExFAT formatted) which I need to keep in sync.  Specifically, drive B must mirror the contents of drive A and drive A must not be modified.  How can I do this without having to do a full copy of drive contents every time?  I'm on OS X 10.10.

This is simply for periodically backing up the contents of drive A.
You could use rsync like this:

rsync -av --delete /Volumes/DiskA/ /Volumes/DiskB


--delete option is for delete files in DiskB that was previously deleted on DiskA.

If you want to schedule it to execute periodically, I suggest you to read How can I run/stop/relaunch an application automatically, at boot/login/some other time?.