From an Applications -> Utilities -> Terminal session you would create a symbolic link as follows
cd /Users/your_short_username/path/to/where/you/want/to/put/the/symlink
ln -s /Volumes/HDD_name/path/to/where/you/are/storing/the/moved/files symbolic_link_name_you_want_to_use
When you view the symbolic link from the Finder it will use the same icon style as an Alias, but it is NOT an Alias. An Alias and a Symbolic Link at 10,000 feet appear the same, but there are differences. For example an Alias is a Finder concept. A Symbolic Link is a file system concept. An Alias can often times find the file or directory even if the file or directory has been dragged to a different location on the volume. An Alias can contain file server connection information for files stored on a shared network attached file system. But the big benefit to a Symbolic Link is that because it is a file system concept, ALL programs can use it without any special support. The file system hides the fact that it is not a real file or directory and just silently accesses the symbolic links target.
The MOST important think about creating the Symbolic Link is that the first path be accurate. You can verify that using the ls command (lowercase L, lowercase S, as in the Unix file system list command). Since you want to put this on an external disk, you need to specify the path via the /Volumes directory and the name of the mounted external drive.
Another way to get the correct path is to use the Finder to find the HDD folder where you are going to move your files, then drag and drop that folder icon to the Terminal session. This will cause the full path to that folder to be pasted into the Terminal. So type
ls -s <space>
drag and drop the destination folder to the Terminal
now you should have
ls -s /Volumes/HDD_name/...
And finally enter the name of the symbolic link you want to use. If you are not in the directory where you want the symbolic link to live, then again use the Finder drag and drop trick to place the Folder path into the Terminal session.
