If you are a seasoned photographer, this is old news.
But, if you are a hobbyist, or even dream about becoming a photographer someday, you NEED to learn this trick now: Create smart previews when you import in Lightroom.
It seems so simple, but it's one of the most essential Lightroom tricks I have ever learned. Unfortunately, I discovered smart previews 25000+ photos into my photography journey.
As I am upgrading my laptop, I just got a reminder of how important they are. Part of my catalog looks like this.
What are Smart Previews?
They are "lossy DNG files, which are a maximum size of 2540 pixels on their longest edge. They are created from the original raw files, and they are stored in a folder in the same location as the Lightroom catalog." For instance, I store my RAW files in an external hard drive and in the iCloud. I also organize my files a lot. So, I know the pain of getting the "photo is missing" message.
The thing is Lightroom doesn't store the actual images.
When you import them into the catalog, Lightroom notes where the photos are located. If the drive is not connected to your computer, then you will not be able to edit your photos because the editing tools get disabled.
If you create smart previews for those images, you will be able to edit them when the originals are not available.
The solution is simple:
You can create them by selecting the Create Smart Preview checkbox in the Import dialog.
They are handy if you are on the go, or don't want to move your external around. Lightroom takes time to build them, and they take up hard drive space, but they can be lifesavers, or "photo savers," in case you delete the original RAW files.