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Yes you can delete your catalogs, but you will lose any work already done on these photos (edits, keywords, collections, slideshows, books, etc.)
If you still want to delete the catalog, just go into your operating system and delete the catalog file (name ends with .LRCAT). Then open Lightroom and it will likely ask you if you want to create a new catalog.
You might also want to keep the existing catalog(s) and just use your operating system to copy them to the external HD.
What have you done that makes you feel that your catalog is all messed up?
I have over 3000 photos on my internal hard drive, but only 285 import into Lightroom. In addition, all of the other photos are "greyed-out" and I can't access them. Finally, when I imported, it put all the photos into folders with the year rather than folders which I have named.