The MacOS like may other operating systems actually sees the path to the file as part of the file name. This is why the : is an illegal character for filenames on the Mac- it's the same as a / on the PC. Since "Enforce Mac/PC compatibility" is enabled you need to follow the rules for PC's on your Mac even though your MacOS allows the use of / marks on names.If you see this error it means that somewhere along the line you have a / being used as part of a drive or folder name. For instance if your saving your session here:Macintosh HD | --Pro Tools sessions | ---- project X / director's cut | --- new sessionThis error will be posted because of the / mark in the name of the "project X / director's cut" folder. Had you saved the session outside of this folder everything would have worked since the / wouldn't be made a part of your session's full path name.Likewise, if your drive's name is "MacOS 9.2.2 / Pro Tools" you will not be able to save a session (with Mac/PC compatibility enabled) anywhere on that drive because the / mark will become part of the full path name of everything you try to save there- including the desktop since it is actually a folder within the drive to the computer.