Im copy and pasting a question another guy had which I have too.
I've tried using both aliases and symbolic links to have Logic Pro X access my sound and loop files from a separate drive.
What happens in both cases is this:
I create an alias (or symbolic link), for example, "Alchemy Samples alias" in a external drive Library/Application Support/Logic folder where the sounds are located from a previous install of Logic Pro X.
I drag that alias into Library/Application Support/Logic right next to the original "Alchemy Samples" folder on my new SSD drive, which is both my new boot drive and where a new instance of Logic Pro X is installed.
I delete the original "Alchemy Samples" folder, leaving only the "Alchemy Samples alias" folder.
I then change the name of "Alchemy Samples alias" to "Alchemy Samples"
So far, so good. When I test Alchemy it appears to access the samples which are now only located on new SSD drive.
BUT….if I quit Logic Pro X and re-open, I immediately get another automatic download of the Alchemy Samples folder and content, which i don't need or want.
It's as if Logic Pro X does not recognize either the alias or symbolic link version of "Alchemy Samples"
Like I said, I've tried this with both aliases and symbolic links, and in both cases, the automatic downloading of additional content occurs.