How to put Desktop on External Drive/Non-System Partition?
You can create a folder hard link using coreutils. Remove the symlink you created and install Homebrew if you don't already have it, then brew install coreutils and use:

sudo gln /Volumes/&lt;partition_label&gt;/Desktop ~/Desktop


(Be careful with removing hard links, since the destination will be deleted if you delete normally. To delete a hard link, use unlink ~/Desktop.)