the simple bit is that if you have the same progs running on both machines then you should be able to copy and paste as you describe. You will only find out by trying it. If the progs cannot open your files you may well find a prog for the mac that can read your files and rewrite them for the mac. You may also find on your pc that you have an option to 'save as' the mac version. You should not need to reformat the external drive but simply file under mac for the mac and file under pc for the pc. Make 2 folders on the drive.