My smart previews folder is taking up 40gb on my internal hard drive (OSX, 2015 rMBP). I want to put them, as well as my backups, on my external. How do I do this?
Why have you created Smart Previews?
Why do you want them on an external drive?
The purpose of the smart previews is to stand in for your images when the images are not available, like on a disconnected external drive.
If you are looking to read the smart previews off an external drive, you might as well use the actual images on the external drive.
That said, the smart previews folder and regular previews folder are located in the same folder as your catalog. To move one of those to an external drive, you need to move all three to the external drive.
Using Finder, drag and drop the *.LrCat file and its matching *.LrData subfolders.