Today’s Question: Isn’t it true that preview files [for Lightroom Classic] need to be on the same drive as the catalog? This brings into play the same issue of available storage space you mentioned regarding an internal SSD drive. Preview files can really eat up a lot of drive space resulting in primary drive filling up. For this reason I have just last week moved my catalog and preview files to an external SSD connected to computer via USB-C. The catalog and previews are now removed from my primary internal drive (SSD). If previews can in fact be in a different location than catalog, would it be preferable to return the catalog to the internal SSD C drive but leave previews on external?
Tim’s Quick Answer: Yes, the preview files for your Lightroom Classic catalog must be stored in the same location as the catalog itself. Therefore, the only way to avoid having your previews consume space on your internal hard drive is to move the Lightroom catalog (and thus the previews) to another drive. Keep in mind, however, that this solution can also cause a degradation in overall performance in Lightroom.
More Detail: There’s no question that the previews file for Lightroom Classic will consume considerably more storage space than the Lightroom catalog. That can have a significant impact on the available space on your internal hard drive, especially if your primary internal hard drive is an SSD drive, which will typically provide less total storage capacity than a conventional hard drive.
Just to provide some context, consider my Lightroom Classic catalog, which contains more than 400,000 photos. The actual catalog file that Lightroom uses to manage the information about my photos is less than 4GB in size. The previews file, on the other hand, is almost 90GB in size.
If you want to move your previews file for Lightroom Classic to a different drive, you’ll actually need to move the entire folder that contains the catalog and related files. That can obviously free up considerable space on the internal hard drive on your computer. However, there may be a performance penalty involved with this approach.
Quite often, an external hard drive will offer slower performance than an internal hard drive. The specific performance you can achieve depends on the speed of the storage media itself, as well as the interface used to connect the storage device. In many cases an internal hard drive will have a faster interface than would be available with an external hard drive. So, if you’re going to use an external hard drive to store your Lightroom Classic catalog and related files, it can be critically important to make sure that external hard drive offers optimal performance. Otherwise, relatively slow performance for your Lightroom catalog can result in a frustrating experience within Lightroom.