This is how I fixed my problem with being able to write data on secondary HDD on Mac/Macintosh.
My problem: I created this folder (SteamLibrary) with starting to download a game first through Steam, then deleting everything and copying over my old “steam apps”-folder. So the “admin”-privilege was already there, but it was set to “Read only” by default for some reason (?).
This tutorial fixes this problem instantly. You can try and change it back to “Read only” to see if the problem still occurs; if you want.