Starting in Lightroom 5, Adobe released something called a Smart Preview. It’s basically a preview of your photo, that’s big enough to let you work on your photos even if the drive they’re on isn’t connected. Here’s a screenshot of the option in the Import dialog.

It’s a great option for people who like to stay mobile because they don’t always need to keep their external drives with them. So what’s the tradeoff? Why not build a Smart Preview for everything? Well, they take up space on your computer’s hard drive (or whatever drive has your Lightroom catalog on it). But how much space? Well, after running some tests, I’ve found that it’ll roughly take up anywhere from 2-5% of the actual photos themselves. So, for example. If you’re uploading 1 GB of photos and you choose the Smart Previews option, you’ll have a file called “YourCatalogName Smart Previews.LRData” that’s about 25-50 Mb in size.
How To Delete Smart Previews
A good practice to get in to is this. Let’s say you’re working on a photo shoot and you want access to work on it even if the drive the photos are on isn’t connected. Well, building Smart Previews would be a great option. But then let’s say you’re done working on that shoot after a few days and don’t need the Smart Previews anymore. Well, you can just go to the top menu (in the Library module) and go to Library > Previews > Discard Smart Previews.