Why do render files/ preview files disapear when i reopen project?
 

Everytime i reopen this project my already rendered (Green bar) files are gone and the bar is back to red bar again. I have to re-render each time and wait again. Whats up with that?

Might be a problem with read/write access on that drive. Right click the folder and go to properties. see if read only is checked... if it is un check it.