Saving movies to an external Hard drive

I'm confused by all the alternatives to saving my projects and/or events. I have a 500 Gig external hard drive on which I would like to back up my material in order to avoid all the clutter and clips on my iMac ( I use iMovie 11) which I can then presumably delete after backing up, but then import again if required from the external H/D. I am just using up a whole lot of space on my computer with all the movie files.

1) Do I finalise the projects before exporting ?

2) Under the "share" option should I use "export using quick time" ? What is the difference ?

3) I realise the importance of keeping the raw video the projects point to, so does the actual EVENT have to be finalised in some way before saving ?

4) I have uploaded a few movies onto Youtube (Restricted viewing) and they are automatically converted to MP4 presumably prior to publishing. Does the same thing happen when saving or archiving to an external HD ?

5) Is there any advantage in converting the finished project to another file type before exporting ?

6) When importing back into iMovie, does it have to be any particular file type ?

7) Is there anyway I can separate the events from the projects when saving externally ?

These are the questions that come immediately to mind, but I would appreciate hearing from others what they feel is the best way to back-up externally on a dedicated unit. I already have time machine, but I find that I would rather have all my movies together on one unit rather than search for them by date when saved with a whole lot of other stuff. I don't really want to burn each individual project onto a disc which is another alternative I suppose. I have lost precious video in the past and don't want to experiment before I know what I am doing.