Need to move previews to external drive.

You would want to consider relocating the catalog onto another drive. Those Library previews, and any smart previews, must be on the same drive under the same parent folder as the catalog.

• https://mastering-lightroom.com/lightroom-classic-hard-drive-space/
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Neither is possible. Smart previews have nothing to do with normal previews and are not a replacement, so you cannot 'convert normal previews to smart previews'. And previews are hardwired to be stored inside the catalog folder, so you cannot* move them to an external drive. If space is an issue, then you can set the size of the previews cache very low in Catalog Settings. You can also simply trash the previews file regularly. Lightroom will rebuild it if you do that, but that takes time so this keeps the size low. Finally you could consider moving the entire catalog folder to that external drive.

* There is a hack that works (or at least used to). If you know what a symbolic link is (it's not the same as an alias!) and how to create one, then you could move the previews file to an external drive and place a symbolic link to the file in the catalog folder. Make sure you only start Lightroom when the drive is mounted, because Lightroom might crash if it isn't.