First thing to clarify is that Lightroom does not get "full". Your images are not in Lightroom. Lightroom opens the catalog which is a database file, and displays your images from the database which simply points to where your images are located on your computer. If your hard drive is that full then I would suggest that you import your images and put them on the external hard drive as part of the import process.
As far as the images that won't import again are concerned, if you go to the library module and click on previous import, does it show those images? This might take a few steps to figure out what's happening. But let's start with the answer to that question.
Yes, I know that. Sorry I worded that incorrectly.
Yes I did try that! Not showing up there
I'm not sure what's going on. It might just be easier for you to create a new catalog and start the import process all over again. Can you find the folder in the library module? Maybe you could right click and remove the folder, then try to import again.
Thanks for your help Jim. I really appreciate it. That didn't work however, I did just figure it out!
Turns out they are all in an external folder called NO NAME that I never noticed until now . Found all of the missing photos in that folder. They were unlinked but I just linked them back up and they all moved to the correct folder after that.