Copy the folders you want to move to the old hard drive. You needn't put them in any particular place. I've created a folder at the root level of my old hard drive, named it My Home Folders, and then moved the folders into it. Now, to Terminal.
Within Terminal enter cd and enter the location of your home folder on the SSD drive. The easy way to do this is to type cd, followed by a space, and then drag your home folder into the Terminal window. Press Return and Terminal will list the name of your home folder followed by a prompt--chris$, in my case).
Now type sudo rm -rf, a space, and then the name of the folder you want to delete. So, to delete the Downloads folder you'd enter rm -rf Downloads and then press Return. Enter your password when prompted, press Return again, and the folder will be deleted. Repeat this for each folder you wish to delete (again, after making sure that you've made copies on the old hard drive).
I'd use this command to delete my original Downloads folder (after first making a copy).
You must now create links to the folders you moved to your hard drive. You can do this by typing ln -s, entering a space, and then dragging in the folder on your old hard drive that you want to create the link to. In our example I would type ln -s and then drag into the Terminal window the Downloads folder from the My Home Folders folder I created on my old hard drive. When I press Return the link to the folder is created within the home folder on the SSD.
Worth noting is that when you delete those folders from the SSD's home folder they'll also disappear from Finder windows' sidebars. You can put them back (minus the custom icon, which you also lose) by navigating to the original folders on the old hard drive and dragging those copies into the sidebar. (If you drag the links from the SSD's home folder into the sidebar the links will simply disappear from the home folder.)
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