Hello and welcome.
-From your explanation, it seems your photos are loose in the Pictures folder?
-How may photos do you have to organize?
-How are the photo files named?
-Which version of Adobe Lightroom?
Without knowing those details, you could always put them all in a single folder, move that folder to the external HD, import that folder into Lightroom, and organize them within Lightroom. It may be easier to do that, since you will have thumbnails of all the photos. I believe Lightroom gives the option of importing them directly into its library and that you can store and access the library from an external HD.
C