Lightroom’s Catalog Previews can take up gigabytes of valuable Dropbox space. But Lightroom’s Catalog is precious and needs to be protected by Dropbox. Adobe provides no simple setting for isolating these two files so here is the workflow I use to keep my Dropbox full of photos and not catalog previews.

1. I start by creating a new directory under the root level of my Home folder Library called Lightroom. (~/Library/Lightroom)
2. I also create a new directory within my Dropbox folder called Lightroom Backups. (~/Dropbox/Lightroom Backups)
3. I then move my Lightroom Catalog.rcat and Lightroom Catalog Previews.lrdata from ~/Pictures/Lightroom to the new Lightroom directory I just created (~/Library/Lightroom)
4. I must then start Lightroom by clicking on the Lightroom Catalog.rcat file within the ~/Library/Lightroom folder I just created.
5. From the Lightroom menu in the menubar I choose Catalog Settings.
6. From the General Tab I set Back up catalog to Every time Lightroom exits, a Lightroom 3 only feature. (While using Lightroom 2 I would set this setting to Every time Lightroom starts, and make sure to restart the app again after quitting to backup my changes.)
7. I then quit Lightroom and choose the destination of my Backup Folder to be ~/Dropbox/Lightroom Backups the directory I created earlier.
8. In order for this to work I must always remember to manually copy my Lightroom Catalog.rcat file from ~/Dropbox/Lightroom Backups to ~/Library/Lightroom before launching Lightroom. (It becomes easier to remember if I do this each time before starting Lightroom.)

Not the cleanest way of separating my previews from my photos, but it works well and allows me to keep multiple revisions of my catalog as Lightroom routinely backs it up upon exit.

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