there are few errors:

1. you should place your code within main() function or not use functions at all;
2. If this is threshold violation event, then instance will be passed as event's parameter #6, and can be accessed as $event->parameters[6]
3. Expression

return (value != null) && (value > 0);

will return 0 (false) or 1 (true) which is probably not what you want.

Script that will return value of FileSystem.Free for current instance or N/A if it cannot be obtained can looks like following:

value = AgentReadParameter($node, "FileSystem.Free(" . $event->parameters[6] . ")");
return (value != null) ? value : "N/A";

Best regards,
Victor

I've done:

sub Free()
{

   value = AgentReadParameter($node, "FileSystem.Free(" . $1 . ")");
   return (value != null) && (value > 0);
}

In a script library.

(I wanted that the script work on the actual "instance" letter Drive, is that possible?)

Then, in event message "%[NameInTheLibrary]" doesn't work.

you can use scripts for that. You can insert return value of script from script library using %[script_name] macro, and in a script you can use AgentReadParameter to read data directly from agent or GetDCIValueByName/GetDCIValueByDescription to get value of another DCI.

Best regards,
Victor