ChornoSync is what I use on my Mac for backing up user data – including my images. Although I don’t use Lightroom nowadays as much as I used to, backing up the database is important. There’s one gotcha, however. Preview images are stored in something called a bundle. A bundle is a directory that OS X handles as a single entity. For the catalog named Something.lrcat the preview bundle is named Something Previews.lrdata. So if one single preview image is changed, OS X thinks that the bundle is changed and ChronoSync happily copies the entire multi-hundred-megabyte conglomerate, not just the changed (or new) previews.
There’s a simple solution to this issue, shown on the image below.
Check the Dissect packages checkbox on the Options tab, and the Previews bundle will be treated as a regular directory, and ChronoSync will just synchronize newly added or changed previews.