Today’s Question: My Lightroom catalog contains about 350,000 images. Since the catalog is quite large, I prefer not having the previews on my main drive, but rather in the external drive. I prefer having the catalog itself on the internal drive which enables me to work on smart previews without the hard drive being connected. Does this present any problems and what is the best way to accomplish this?
Tim’s Quick Answer: While there is a way to store the previews file separate from the Lightroom Classic catalog, doing so is a little complicated and won’t completely solve the issue you’re trying to address.
More Detail: By default, the preview cache for your images is stored alongside the Lightroom Classic catalog file and related helper files, all in a single folder. That means the catalog, the previews, and the smart previews, for example, are all in the same folder and therefore on the same hard drive.
It is possible to store the previews file in a location separate from the Lightroom Classic catalog and still work normally in Lightroom Classic. However, in order for this to work you would need to have the previews file available. In other words, in this example you would need to have the external hard drive connected in order to work in Lightroom Classic, since the external hard drive would contain one of the key files required by the catalog.
Since in this case the intent is to be able to work in Lightroom Classic with the smart previews available but with the previews file on an external hard drive that may not be connected, there is not a workable solution.
Therefore, I recommend leaving the preview cache and other related files right where they are in the same folder that contains the actual Lightroom Classic catalog file. For those who are interested in moving their previews file, or are just curious of the technicalities involved, you can read on. Otherwise the rest of my answer may not be of much interest.
If you wanted to store the previews file in a different location (such as an external hard drive) that would be available whenever you’re using Lightroom Classic, you could do so by creating what is called a “symbolic link” to the file. This is a form of shortcut in the location where the file is expected (alongside the catalog) that links to the actual location of the source file.
I don’t really recommend this approach unless you are very comfortable with command line processing within the operating system. If you want to proceed, I suggest creating a good backup of your existing catalog files first. You could then move the previews file for your catalog to the desired location.
To create a symbolic link on Macintosh you would need to go to the Terminal application found in the Utilities folder within the Applications folder. You can then use the “ln” command (as in “link”) to create the link, with a syntax like this:
ln -s “/Volumes/VOLUMENAME/Folder/Lightroom Classic Catalog Previews.lrdata” “/Users/USERNAME/Pictures/Lightroom Classic Catalog”
The above command assumes the actual previews file is on an external hard drive called “VOLUMENAME” and that the catalog folder is within the Pictures folder. The VOLUMENAME and USERNAME references would therefore need to be replaced in the above command with the appropriate values.
Windows users who are interested in taking this approach should also obviously backup their catalog first, and can then refer to this article for instructions for creating a symbolic link with the Windows command prompt: