a couple of ways you can do this:
One is to boot without a startup-sequence by holding both mouse buttons when the computer starts. At the command prompt, simply type each of the lines of your startup-sequence manually, one by one, until you find which one is triggering the failure.
Another is to edit your startup-sequence and remove all the instances of ">NIL:", "NOREQ", and "QUIET". Just make sure you backup the file before you edit it. Boot the computer with the modified file and you will see the output of each command and, theoretically, the output of whichever one precedes the failure you are trying to isolate.
Another is to output everything to a file rather than ">NIL:", but this is a little bit more complicated. Try the other two things first.
