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've never seen a Mac with two external FAT disks :) Out of curiosity, what hardware are you on?
    – Gabor
    Nov 9, 2014 at 8:04
  • @ber4444 There's nothing notable really, I just have an external hard drive which I need to be readable and writable by both OS X and Windows. I need to periodically back up the contents of this to another hard drive. They're not connected to the mac all the time.
    – Szabolcs
    Nov 9, 2014 at 17:33

3 Answers 3


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.


Use 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.

  • Both hard drives need to contain identical data on identical file systems so this solution is not acceptable. Also, Time Machine doesn't exactly do what I need: simply keep the drives in sync. It tries to preserve edit history and does other things that are not necessary and undesirable (to do the unreliability the additional complexity brings and the extra space requirements)
    – Szabolcs
    Nov 9, 2014 at 21:45
  • then you need to copy byte by byte and deal with all the lost hours instead of dealing with some extra space
    – Gabor
    Nov 9, 2014 at 21:54
  • I am looking for software that will compute the difference and only update on HDD B what changed on A.
    – Szabolcs
    Nov 9, 2014 at 22:06
  • The thing is that I did find such software (e.g. this), but most of what I found is not free. I was hoping to either find a free one or if I end up having to spend money then have some assurance that the software will work well ...
    – Szabolcs
    Nov 9, 2014 at 22:25
  • from the link it seems you want not drive backup but dropbox-like sync without the cloud, so why don't you use bittorrent sync (getsync.com/how-it-works)
    – Gabor
    Nov 9, 2014 at 22:37

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).

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