What is the path to the files when they are on the external drive? Any Unicode characters?
Greg:
I nearly got an answer yesterday and this goes on the file path.
In the external hard disk, the path to the project hods ", (, ), a folder with leading spaces (3 or more), numbers, etc.
No, I do not think there is something like UTF-8 characters (probably only ASCII characters).
Last note: I moved the project holding (parent) folder into the hard disk root and the problem disappears (nothing else was done).
The project name have ASCII standard name (no number, a character as the first character of the folder name) so is the project name.
I do not changed the project parent name to check which character is bad (parenthesis, double quote, leading space, etc.).
I have a text file that explain a bit more how I go there, but above is the best part of it (from memory).
Remember: as far as I know, I do not had to make changes to the project file to get the error dialog (watch it at the start of the page). I only had to make a copy of the project to my internal hard disk, eject the external one and OK.
A boot on El Capitan external Hard Disk goes on the same direction (a copy of the project folder into the El Capitan Hard Disk works fine).
Tell me if I can do some more tests.