Well, if you're talking about FPCX, there are a couple of choices that can make a difference.
In Preferences, Import>Organizing you have a choice of copying to Events folder or not. I don't because I want to have control over the process – when what and where copies are made. (If you import from a camera, however, it will automatically make copies…which is desirable). So we're really talking about whether to copy files already on your HD. (I don't use FCPX to import from the camera, but that's just me holding on to some legacy work flow.)
Under Transcode, you have the choice to create optimized (Pro Res) or Proxy (Pro Res Proxy). I prefer to leave those unchecked because I'm often going to use only a small fraction of what I import and I don't want to create a lot of big files that I'm later going to have to prune. After I figure out roughly what I need then I do any transcoding from the Events browser.
You also have these Organizing/Transcoding choices when you create a new event; when you do, a dialogue box pops up.
Something else to pay attention to is your render files…which may or may not be on your boot drive. They can accumulate rapidly butcan be managed either from within FCPX or in the finder.
Russ