Accessing internal hard drive from a bootable SSD

Hi - my 2013 MacBookPro is running 10.13. I attached an SSD with 11.7 on it and booted from it with no problems. I'd like to move a few documents from the internal drive but the folder for the internal's Desktop has a minus sign and when I double click on it - "you don't have permission to see the contents".

How can I gain access to the internal hard drive while running from the bootable external so that I can copy documents from the internal to the external drive?

TIA your response.