Record long webcam video / avoid re-encoding / save video-capture instantly I want to record a very long video, approximately three hours, over an external webcam.

Short videos work very fine with QuickTime and its built-in "New Movie Recording" function. As long as they are short, you can choose to save the file "unchanged". Apple calls this to save the file "[…] in the same format as the original".

The option "save a movie file that contains media in the same format as the original movie" mentioned above strangely disappears at my three hours video. 

My webcam needs 180 MB disk space per minute in 1080p, so a three hour video roughly weights 30 GB.

This export takes a quite, quite long time and I do not want to re-encode it and also want to save it in the same format as the original movie.

Another question is if there is a possibility to save the file instantly while recording? I am afraid of losing my video if my Mac crashes somehow. Also iMovie is not capable of this. Requirement is that I want to save the file "as it is" in 1080p, or how Apple calls this "in the same format as the original movie" and not down scaled or anything like that.

I appreciate every help concerning QuickTime or iMovie and also suggesting of any other applications.

Here are some screenshots. The second and third one are triggered either by closing the file or choosing "export" from the file menu. The option is ONLY available for small videos. For my large video there are options like "audio only", "Mac and PC", "480p", "720p", "1080p" and so on.
Quicktime can't encode and capture at the same time that's why the files produced are so big.

Adobe provide Adobe Flash Media Live Encoder free.
You should try with it.

Be aware that encoding live 1080p take a lot of ressources.