Move Lighroom Catalog Previews.lrdata file to eternal hard drive (MAC)

I have to manage the same thing because my MacBook Pro internal storage only has so much space, but I like keeping the catalog folder on it. (The 150,000+ originals are on an external volume.)

In the past, if the previews cache file got to be over around 40GB, I would just delete it, and Lightroom Classic would start over from zero and free up the space. So my preview cache never got to be larger than 100GB. The disadvantage of this method was that because I threw out all the previews, even the recent ones I still needed for current work had to be rebuilt.

Today, I use the new option that limits the size of the preview cache. This is great, because I no longer have to manage this manually. I tell it what I want the upper limit to be, and it keeps it down. You might notice that it says the preview cache file size is 21GB even though I entered a limit of 10GB. That might be because of a line in the help article for this feature: “Previews that are 30 days or newer are never deleted.” I’ve browsed a lot of folders recently while looking for images, so I’ll bet some of the excess previews were generated for those; they’ll probably be auto-deleted over the next few weeks. But you know, 21GB is a heck of a lot better than the 30GB or 40GB it used to become before I would throw out the whole preview cache in the past, so this is still an improvement.

If you don’t think that’s a good solution for you, and you still want to move the preview cache to another volume, the only practical way to do that is to move the entire catalog folder (the catalog and all of its associated files). That’s because the catalog always creates its preview cache in its folder. 

If you want to slow down how fast the preview cache grows, one thing that helps is to lower the Standard Preview Size and Preview Quality in the same Catalog Settings / Previews window. If Lightroom Classic detects that the main display has a very high resolution, the default Standard Preview Size of Auto might be much larger pixel dimensions than you’re expecting, causing the preview cache to grow faster, and causing preview generation to use more system resources and take longer than you want. I decided 2048px was enough for me. (I can always manually generate 1:1 previews for any images that need it.)

The preview cache has to be in the same folder as the catalog, so you will have to move the whole folder to the external drive, which will probably affect performance.

You can delete the preview cache (or move it to the external drive, which will have the same effect), and LrC will then rebuild the previews as you browse folders in Library. Folders you don't visit will not have previews built until you click on them.

You can't move the previews database alone to the external drive. You'll have to move the entire catalog to the external drive. I would recommend that you use an SSD for this, and not an HDD.

Alternatively, you could limit the size of the previews database. That's available in the Catalog Settings (Opt-Cmd-Comma) on the "Previews" tab.

Better you should simply delete some of the older previews. In LrC, select some (many or all) photos and use the menu command Library->Previews->Delete 1:1 previews. (And if you have smart previews, delete teh smart previews as well).

Moving the previews to anothe disk is technically possible, but then you lose the benefit of previews being on your fastest disk, and your Library module will slow down. Of course, deleting previews causes LrC to be slower also.

Moved from the CC Desktop forum to the LR forum.