I'm not sure that I completely understand you, however...
70GB is quite small for a system drive, by the way.
I never save video recording files to system HDD, I save to a separate partition which is reserved for all data files.
After installing Debut you can choose where to save your recordings in the Options, under Output (but my version may be set out differently from yours, as it is older - still, you should find something similar).
Make a new folder for your videos on the drive that has space.

Also, check the format that you are recording in. If you are using "fast capture mode" the files will be enormous. I use mp4 as it is easier to edit the recordings; avi would be slightly better quality and bigger files.
If my reply is not relevant to your problem, just ignore me 