Lost file during QuickTime recording

QuickTime Pro offers a convenient way to record a video or audio file, for example using the built-in camera present in all recent Macs.
But the recording process can fail for a number of reasons, like battery failure or lack of disk space. In most cases, you can recover the movie following a few easy steps:

The first and most important thing is to prevent the file from being deleted by the system. As it’s a temporary file, rebooting the computer could delete the file.

Do not reboot your computer!

We will find the file and move it to a safe place. The location of the file depends on the volume on which you were recording:

In Boot volume:
Boot volume is usually your internal hard disk. It’s the disk that contains the operating system and your home folder.
In this case, the movie file is located in /private/var/folders/

This folder is not visible in the Finder. It’s hidden because you are not supposed to store your data here. But there’s a way to explore this folder anyway:

From the Finder, use menu Go > Go to Folder … and type /private/var/folders/

A Finder window opens, revealing a few subfolders. Now you can search inside, until you find one or several files with extension .mov and whose date of creation matches the QuickTime recording incident.
Just move those files to the Desktop. Once on the Desktop, the files are safe.

In an external drive, or a volume that is not the boot volume:
In this casem the file should be in this folder:
/Volumes/nameOfExternalDisk/.TemporaryItems/folders.501/TemporaryItems/


Note the . before TemporaryItems, it makes the folder invisible in the Finder.
You must use Go > Go to Folder… and type the path like this:
/Volumes/nameOfExternalDisk/.TemporaryItems/

Do not forget to replace nameOfExternalDisk by the real name of the volume!

Then explore the subfolders until you find the .mov files whose creation date correspond to the QuickTime session, and move them to a new folder on the same volume.

Now the files are in a safe place. You can reboot your computer if needed.

As the recording process was interrupted, the files are not playable: QuickTime gives an error message when you try to open them.
Fortunately, there is Treasured, and you can diagnose and preview the file.