How to set the same storage service for all sites?

This has to be done by App Manager.

To set up storage service for all sites:

1. Click on your avatar at the upper right hand corner, go to "Manage Application".

2. Go to "Setup" > "Storage".

3. Select "App Level" under "File storage level:" to make all sites use the same storage service. Then you can select which one you want to use in the "File storage location: " drop down menu.