I am still a Mac beginner, but it sounds like the same thing that happened to me, and was because the permissions somehow got changed. Did you check to see if you have “Read & Write” access?
Open Finder, right click on the drive in the left column and select “Get Info”.
Go to “Sharing & Permissions” and make sure you have “Read & Write” privileges.
If “Read only” is listed, click on the lock and enter your password when prompted.
Click “Read only”, change to “Read & Write” and click the lock again to save changes.
I hope this helps!