I am getting this message - Could not save as ... because write access was not granted
That sounds to me like a file permissions issue:


Check if the file you are trying to open in Photoshop is currently open anywhere else (ie. another program is using it, hence locking write access).
Right-click the file in Finder and check if your user account has full access privileges to manipulate the file, if not, add them


Hopefully those two hints may get you closer to your desired outcome. Let me know how you get on.