This appears to be more a Mac OS X 10.3.9 problem than a Final Draft problem; the Apple support page returns 150 articles on this error and it affects many different programs.
We have not been able to replicate it on any of the 20 or so Macs in our office and therefore have not been able to run tests, so the solutions offered below may or may not be effective.
If you're not able to save the script after an Error -43:
If you're able to save the script, click HERE.
NOTE: Live support for Final Draft 7 has been discontinued. You will still be able to install, activate and use FD7 on appropriate computers but email, chat and telephone support is no longer available. Live activation assistance is still available via phone (818-789-6281) and email and chat (links on the left side of this page).