Photoshop on mac "could not save [file] because write access was not granted"
After some trial and error, I tracked it down to the file being selected in Finder. 

It seems like when Finder is displaying a preview of a PSD, it locks the file. 

It seems to only happen in this view mode:



Selecting another file in Finder, or switching view mode, or closing the finder window, all prevent this error.



So while Finder is like this, the file won't save:



Clicking off it frees it for saving:



As does switching to this view mode to one that doesn't show a preview: