The idea of moving your images to an external hard drive to free up disk space on your main hard drive is a good one. You can do that without the need for creating smart previews. The only time you need smart previews is when/if you are going to need to have access to the images with the external hard drive disconnected. Then you can work with the smart previews, which would still be on the main hard drive, and the work you perform on those smart previews would have effect on the master images as well.
The best way to move your photo collection to an external hard drive, in my opinion, would be to copy the entire photo collection to the external hard drive. Then, in the library module, right-click on the topmost folder in your catalog and choose the option to update the folder location and point Lightroom to the folder on the external hard drive. So, ideally, if you have your photos organized so that they are all under one master image folder you would only have to do that exercise once and Lightroom would be reoriented to seeing the images on the external hard drive. Once you have verified that Lightroom is functioning properly you could safely remove all the images on the internal hard drive. Whether or not you need to create smart previews is something you will have to determine. One thing to remember is that when working with smart previews the option to edit in Photoshop is not available. In order to do that the master images must be online.
If your images are not organized under a single master folder then you'll have to repeat that update location process for each folder, but that only takes a few seconds for each one. Not really a big deal.
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