
Okay, this isn't specifically a Final Cut tip but since you may need to occasionally scan in things for FC you may find yourself inside of Apple's free Image Capture app.
If you do, and you find it working perfectly except for it's Scan button is greyed out, here's the solution:
Click in the preview window and drag out a square for it to scan from.
Yes, that's it. It's that simple. Click outside the scan marquee square you just made to start over. This had me puzzled for far too long.