i moved some pro tools session onto an external hard drive, then tried to move them back to my mac and open them up, now i get an error message, the audio files play individually but the session won't open is there a way to fix this?

the session is not finding the audio files in their original directory. There should be a pop up window that asks if you want pro tools to find the file automatically or if you will find it yourself.

First of all, you shouldn't be opening sessions on the same hard drive that your protools application is installed on. You can get a way with it for a minute, but over time it will really screw things up. You should be opening/creating sessions on a second high speed drive(i.e. external fire wire with 7200rpm bus speed). As for your question, cosign the above answer. Otherwise, sometimes protools gets inexplicably picky about recognizing files, especially a file created on another system, hard drive, or protools version. This doesn't happen too often but it could be the case here. Unfortunately, if this is true, you might have to create a new session and import the files and rebuild the thing. Tedious, yes.