Are you using footage from an AVCHD camcorder? If you are, you should be using Log and Transfer, not Log and Capture. I would never use iMovie to import footage for FCP unless ABSOLUTELY necessary - iMovie uses AIC for its storage codec which will store 1920x1080 as 1440x1080 (anamorphic) - which means you are loosing quality right off the bat. When you import with FCP, you'll be storing it in ProRES, which does take a lot of space (more then AIC) but is an incredible codec for editing.
You need to use a drive other then your system drive for your scratch and render space (you should NEVER use your system drive as a scratch drive) - depending on the type of Mac you have will determine what kind of extra drive you can use (ie: Mac Pro - internal sata drive, iMac or Macbook Pro a firewire 800 drive, Older Macbook - firewire 400 drive, more recent Macbook - USB drive) - you will want to transfer your footage to the new drive, then in FCP, go to FCP->System Settings and set your capture and render location to be the new drive.