OK, I used Tools | Global Options | General | R Sessions | Default Directory to change the location from ~ to ~/Desktop, and it works.
The change is written to the file /Users/ME/.config/rstudio/rstudio-prefs.json where it appears as "initial_working_directory": "~/Desktop",
The relevant permissions are
-rw-r--r-- 1 ME staff 303 Feb 11 01:11 rstudio-prefs.json
drwxr-xr-x 4 ME staff 128 Feb 1 23:52 rstudio
drwxr-xr-x 4 ME staff 128 Feb 1 23:52 .config
The .config directory in your home directory should have been created with the installation of RStudio.
If it's missing, use the terminal
[your prompt] mkdir .config
[your prompt] cd .config
[your prompt] mkdir rstudio
[your prompt] ls -la # check directory permissions for rstudio
[your prompt] cd ..
[your prompt] ls -la # check directory permissions for .config
Make a change to global options in RStudio and
[your prompt] cd ~/.config/rstudio
[your prompt] cat rstudio-prefs.json
You should see the changes reflected.