Quicktime, "New screen recording" and recording length

heres the problem: my HD only has 450mb of space left and l like to record clips of personal videos with quicktime "New screen recording", some videos are 20mins long, I realise that the HD would never have enough space, so l have directed QT to save in my buffalo 1TB external drive, yet it automatically stops after ~7mins, "operation stopped there is not enough available space to continue the writing. make room by deleting existing videos" , my buffalo 1TB has max space. how do l record the videos without having to spend hours transferring files and folders from mbpr HD to 1TB buffalo external ? (very importantly is this manual transfer necessary since time machine is set up and progresses and completes often whenever my other 1TB touro drive is connected or do l still have to manually transfer important media files etc to my 1TB buffalo?)

I realize you've told QuickTime to save the video to the external drive but if you actually have only 450 MB of space that's not going to cut it. The problem is that there isn't enough room for the various temp files/swap files the OS uses. I also suspect that QuickTime stores a temp file of your recording until you actually save the file to the external disk. If that's the case 450 MB really on't cut it.

Are you experiencing similar errors in other memory/disk intensive programs? It also wouldn't surprise me if you weren't seeing other slowdowns systemwide.