Inspiring

Answered

Lightroom Classic 15.3.1 on iMac OS 15.7.7 (SS 2TB with 824 GB available) does not display thumnails until they are selected. lrcat size 4 GB. Previews.lrdata 29 GB

Forum|Forum|22 days ago
June 30, 2026
4 replies
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It’s not that the thumbnails are slow to display; they never display until they are selected. Images are stored on an external drive. The catalog is on the iMac SS.

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Correct answer JohanElzenga

Unfortunately, yes. You don’t have to do that manually, however. You may not view a lot of your old images anymore, so there’s no need to generate previews for them. Leave it to Lightroom. It will rebuild previews only if and when needed.

By the way: if you have 700,000 images, and the previews are only 29GB, couldn’t that mean that you caused this problem yourself by setting the preview cache size in the catalog settings way too low?

lowellnelsonAuthor

Inspiring

Greetings Johan

I’m still a bit confused. If I understand you correctly Lightroom will build missing previews only as images are selected (“as needed”) on-screen, even if the Previews.lrcat file is being rebuilt. This seems to contradict other advise (eg PetaPixel, eg Tim Grey) that indicates that Lightroom will rebuild the previews in the selected folders without my having to select any individual images, even if the process takes hours or days.

JohanElzenga

Community Expert

Tim Grey is wrong if he said that, but you are indeed a bit confused. You do not have to select an image before Lightroom builds a preview, the image just has to be visible in the grid. That is what I meant with “if and when needed”: they are needed to display the image grid. For images that are not visible, Lightroom will not automatically build previews. You can force Lightroom to build previews for all images however, by selecting all images and then choosing ‘Library - Previews - Build Standard-sized Previews’, so perhaps that is what Tim meant. And that can indeed take a long time!

-- Johan W. Elzenga

lowellnelsonAuthor

Inspiring

Yes, Johan, that is the current status of the question: If the Previews.lrdata is rebuilt, will it be done continuously in the background until complete, or only as images are viewed?

lowellnelsonAuthor

Inspiring

@JohanElzenga

Thank you Johan.

I realize that I actually haven’t fully described my problem: some thumbnails are displayed; some are not. For those that are not, when the cursor is clicked and held on a blank “image” for half a second or so the thumbnail is created and displayed; otherwise, I can wait until the cows come home without further progress, even if the selected folder contains only a few images.

This behavior remains the same when the previews.lrdata file is removed and Lightroom rebuilds it.

JohanElzenga

Community Expert

I have no explanation for that. Maybe there is a certain edit that stops Lightroom updating the preview, which would explain why some images have that problem while others don’t, but then I would still not know why.

-- Johan W. Elzenga

lowellnelsonAuthor

Inspiring

Well, I haven’t set a limit to the Preview cache size.

JohanElzenga

Community Expert

Then most of your older photos probably have no previews, because 29GB for 700,000 photos is very little. Anyway, it doesn’t matter for the problem and the possible solution.

-- Johan W. Elzenga

JohanElzenga

Community Expert

Your previews.lrdata may be corrupted. Try this: move the previews out of the catalog folder and then start Lightroom. Lightroom should rebuild them, so you should see them appear one by one.

-- Johan W. Elzenga

lowellnelsonAuthor

Inspiring

If that turns out to be so, does that mean that previews will have to be rebuilt for all of the 700,000 images in my database?

JohanElzenga

Correct answer

Community Expert

Unfortunately, yes. You don’t have to do that manually, however. You may not view a lot of your old images anymore, so there’s no need to generate previews for them. Leave it to Lightroom. It will rebuild previews only if and when needed.

By the way: if you have 700,000 images, and the previews are only 29GB, couldn’t that mean that you caused this problem yourself by setting the preview cache size in the catalog settings way too low?

-- Johan W. Elzenga

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