You indicated that you changed the permissions to write. I don't know anything about the Mac operating system, but you need to have both read and write permissions on the destination folder.
As I stated in the other thread, changing permissions didn't work because (a very common error) you have changed the permissions on the wrong folder. Show us screen captures of the Destination section of the Import dialog box, and the permissions on that exact folder.
I found a solution by creating a new destination folder next to the first. I wonder if there is a limit on the number of items in one folder. It's 21,000 + files.
I'm sure this will resolve itself when I upgrade my aging machine.
Thanks for your help.
As far as I know, there's no limit to the number of photos in a folder (maybe there is on Macs, but I haven't heard of such a thing).
However, it is most definitely not a good idea to put 21,000 files in one folder. This just causes your operating system and Lightroom to slow down at times as it has to crunch through a huge amount of information whenever it needs to do anything with that folder, such as display the files in the folder.