If you are using FCP X for editing you can import from the camera card just the parts of the video you want so not as to clutter your HD up with great long segments of unwanted footage.
(At the moment with FCP X) Once you transfer your footage to the computer HD you cannot easily delete these unwanted segments.
In short. If you use FCP X then leave your camera running, then select just the footage you wish to keep and only transfer that onto you hard drive, reformat your SD card and start again.