We cannot know. You are not providing any relevant details like operating system, what version of Quicktime, how your drive is formatted, security settings, user permissions, exact render settings and so on. Your info is far too vague. "Unable to open file" means that another tool/ program is blocking access, but we realyl cannot know.
I apologize for the lack of information.
the internal hard drive is formatted in NTFS, then I tried a hard disk external 2.5 "formatted in exFAT.
The operating system is Winsows 7 64bit
processor: i5-750 to 3366 MHz
RAM: 8 GB
video Card: ATI HD 5750
I wanted to remember that this only happens with mov files, with other files there is no problem
FAT disks can never have files larger than 4GB, this could be the sole problem here. Other than that really make sure you set the user privileges/ file permissions properly.