This sounds like an issue with your User Data location that you have set in Studio One. By default, the User Data folder is set to Documents>Studio One. This is set in Studio One under Studio One>Preferences (Options on Windows)>Locations>User Data. Inside of the Studio One folder, there is a folder named "Templates." This is where your custom templates are saved to. Inside of the Studio One folder, there is also a folder named "Songs." This is where your songs are saved to. If you open Studio One and click on the "Songs" tab on the Start page, the songs that are displayed are the songs that are in your Studio One>Songs folder.
The bottom line is that you will want to check your User Data location inside of Studio One and then verify that the location is accessible.
Here are a couple of reasons as to why you might have a problem writing to a drive:
1. Make sure that the drive is indexed. Here is an article from Apple that explains how this is done.
2. If a drive is formatted with the NTFS file system, El Capitan and Sierra will set the drive to "Read-only" and you will not be able to write to it without 3rd party software such as "Paragon." Reformatting the drive would be the best solution.
3. If you have your system drive formatted as "case-sensitive," then you can run into scripting errors in Studio One that can cause all kinds of issues.
*Should you be in a situation in which you need to re-build a User Data location, you will want to have a Studio One folder somewhere on a drive. In that folder, you will want to have a "Songs" folder that contains all of your song folders. Once you save a template, the "Templates" folder will automatically be created. In Studio One, you can go to Studio One>Preferences (Options on Windows)>Locations>User Data and point the program to the "Studio One" folder.