editing from captured Master clip

what i think i want to do is capture a lot of what i've shot and whittle it down from there. i have a Lacie external drive that will hold 180gb right now. can i simply capture an hour of video, make subclips, or something (the stuff i want to keep) and then delete or erase everything from the hour that i didn't use? maybe this is a stupid way to go about things but i don't know how to get rid of the rest of the Master that i don't want.

thanks