Lightroom classic- back up photos from one external to another

"Not sure why this should delete the folder structure ?"

Your backup on the external drive needs to be an exact copy of the primary files you have imported into LrC. The easiest way to do this is to use file synchronization software, which will keep track of new files added, files deleted, files renamed and folder structure changes you've made from inside LrC. No changes should be made to the files or folder structure on the backup or primary working drive from outside of LrC. This will cause it to lose track of the files since they are only associated inside LrC using your LrC catalog and NOT physically to the file system. We get many posts from users who have moved, deleted, or renamed files and folders from outside LrC using Finder or File Explorer. They are then  surprised and bewildered when all of those files and folders show as "missing" inside LrC. Don't do that, which is what I was trying to communicate. Enjoy!

"When viewing the image on the back up external, via lightroom, the edits where not there."

As others have mentioned I'm not sure HOW you were veiwing the backup copies in Lightroom. Regardless, don't do that! Backup copies should NOT be reimported into Lightroom or any other editor. Doing so you run the risk of accidently deleting or modifying the files or folder structure. This will cause those files to go missing inside Lightroom when you try to use them for restoration puposes after  a catstrophic event (HDD crash).

At the link I provided:

If you’ve saved all of those files and you ever have to restore from your backups, you can simply return those files back to their correct locations, open your catalog, and carry on working as if nothing has happened.

all photos are on an external HD. Would like to have a back up, including the editing on a second HD without using a third party app.

As mentioned, since LR never makes changes to the original image, all changes are stored in the LR Catalogue (and XMP file for RAW if enabled). So all you have to do is backup your images to a 2nd drive. If you don't want to use a 3rd party, you may have to get a little technical. I have a nightly backup of my LR Catalogue and images using ROBOCOPY on Windows 10. This is a command line utility. I understand that on the MAC the equivalent is 'file sync'.

Tried to copy some photos first-then backed up thelightroom catalogue on exiting to the back-up HD, but no edits in those photos.

I'm not sure how you were trying to see the edits. As mentioned above and elsewhere here, the images themselve are not changed. You see the changes in LR but will not see them externally until you export or print them.

Here's a good article on what LrC items you should be backing. This will allow a full restoration of LrC in the event of a disk failure.

https://www.lightroomqueen.com/backup-lightroom-files/

Lightroom Classic does not store the edits with the photos. The edits are stored in the Catalog. So what you are seeing is expected.

Would like to have a back up, including the editing on a second HD without using a third party app.

I think this is a big mistake, backups should be made at regular intervals and automated. Humans get busy, humans get lazy, humans forget — and then one day your hard disk dies and your last backups is 2 months ago. Use a third party app, set it and forget it.