The other day, almost without thinking, I hit the “install” button on software update, and upgraded to 10.3.5. Since then, I can NOT run Final Cut Pro 4.5. It always unexpectedly quits on launch.

I had to switch system disks to get it to run.

See this thread for others having the same issue.
FCP HD Crashes on launch

Note: 2 days later, problem solved, thanks to this technote from Apple

1. Open Terminal

2. Type

sudo update_prebinding -root / -force

3. Press Return.

4. Enter your password when prompted.

Restart your computer.