Dear friends I appreciate very much your advices and wish to express my sincere thanks to all of you who respond.
I try to use Avid Media composer for professional editing. My biggest problem is that when I work in one computer and want to use the same project on another computer it gives me media offline and actually can not yet use Avid - otherwise it is an excellent software - better organized than Adobe and more stable for bigger projects.
I tested private, shared and external options in c:/ - which I do not think is good and in external disks. I could not catch the difference - in some cases the project was accepted but in some not. Still a puzzle?