New Macbook Pro M4: Catalog size + location?

But then I was a bit shocked when I checked my catalog size. It's definitely taken up MOSTLY by the Previews+Smart Previews lrdata (407 GB)

I think my LR settings are to not discard previews, so go figure it would grow to such a size... but also that's part of the performance; having the previews there for you? I believe I can change it to discard after i.e. 30 days but I don't think I can limit it by size?

By @Kaitlyn2004

Yes, part of performance is having the previews already available. However, previews need to be updated for any images that are edited, so I’m less concerned about having previews around for images I’m currently working on.

Yes, you can (in theory) limit the size of the preview cache in the new version (14). In Lightroom Classic > Catalog Settings, there is a new limit option that wasn’t there in earlier versions, shown below. However, there are some reports that it doesn’t always limit as expected, and as you can see in mine, I’ve had the limit set to 10GB for weeks but the total preview size is still 34GB so I’m not sure what’s going on.

I also use a MacBook Pro where images must be stored on an external drive. Before Adobe added that new preview cache limit, I would simply throw out the huge previews file when it got above 30–35GB. Yes, that means those previews would need to be rebuilt, but for some reason that doesn’t bother me much.

As you can see in Catalog Settings, another strategy to slow down the growth of the previews cache is to lower the Standard Preview Size and the Preview Quality. If you have one of today’s high resolution computer displays and Standard Preview Size is set to Auto, in some cases that can generate previews with pixel dimensions much larger than you really need, taking up a lot more storage than you expect. I decided that I don’t need Standard previews to be more than 2048 pixels long. Because if I want to see it at full resolution, I’ll just switch into the Develop module.

I suggest you edit your post to remove the advertising.

As far as the LrC catalog and previews location goes, it should be on the very fastest drive in your system and in the M4, that would be the internal SSD. For the smart previews, you can delete all but the ones you think you will need while not connected to the drives where your images are located. Definitely set the preferences to delete 1:1 previews older than 30 days. Do that and see how much room those folders take after deletion. You might find that it would fit on your 512GB internal drive. For my M1 MacBook pro, that I use only for travel, I have a 2TB internal drive since I import photos to that drive and backup to external drives.

I like to create a folder for the catalog and previews at the root of the drive so it's easy to find and backup in addition to the LrC catalog backup function.