I can't tell you how to recover them because I don't know what really happened in your copy process. But I have to ask why you think you need to copy a folder of images from the external hard drive to the Mac hard drive to work on them. That doesn't accomplish anything. The catalog is on your Mac hard drive. And that is where all of the adjustments made in Lightroom are stored. It doesn't matter whether the images are on the external hard drive or on the Mac hard drive. They remain completely unmodified and in the same state they were in when they were downloaded in either location. So if you can work on the images with the external hard drive connected then there is no need to copy the images to the Mac hard drive.
To recover the photos, you might want to try something called an "undelete utility program". There are free ones available on that Internet thing. download it, install it, and then run it. Try to undeleted the files from both the internal and external HD. No guarantees it will work. If you are going to do this, do not perform any other tasks on your computer except find it, download it, install it and run it.
With regards to your workflow, I would like to suggest a major improvement. there's no need to move or copy photos from your external HD to use them in Lightroom. If the photos are on your external HD, you can import them into LR using the ADD option, the photos then remain on your external HD but are available in Lightroom for editing and other tasks. If the photos are on the camera card, iimport them using the COPY option and have the Import place the photos on the external HD. This also means you don't have to remove the photos from your internal HD after you use them in Lightroom, and so there's no immediate need to export the photos and store the metadata in XMP.
I hope you will implement a backup routine that includes making regular and automated backups of your photos and your catalog file. The backups MUST go to a different physical disk than your originals.