i just ran into the same problem. here’s what i did:
i saved a copy of the ‘corrupt’ fcp project file to my desktop. then i turned off the external drive that has the files and the original fcp project (that won’t open). i then opened the fcp project that i’d saved onto the desktop. it opened fine (of course, it’s asking me to re-link the files that are missing which of course i didn’t do). now the project is open and i can get to the last sequence. from here you can do a few different things:
1. go in and highlight all of the files in the sequence and copy them. create a new project and paste the files into a new sequence. save a copy of the new project and then turn on the drive with the files on it, open the project and it should work fine. of course, this is the “quick fix” and will create other problems down the road but, if all you need to do is get to that last sequence so you can output it this will work. NOTE: i tried this and it worked. you could ALSO try copying everything from the ‘corrupt’ project into the new project (just drag all files across from the browser into the browser of the new project and save it as a new project-different name). i haven’t done this so it may not work (because you might be copying over the problem that’s making the ‘corrupt’ file not work) but #2 below isn’t much different and it worked. if it doesn’t work you could try adding one thing at a time until it doesn’t work and then you will have isolated the problem.
2. what i did: i opened the replica ‘corrupt’ project that i’d created on the desktop (with the external drive still off). i also saved a replica of a previous version of the fcp project onto the desktop (that wasn’t corrupt but was a bit out of date). having both opened, i copied over the newest sequences from the ‘corrupt’ fcp project to the older fcp project and attempted to update the older fcp project as much as possible looking at the ‘corrupt’ project (copying over links to video files, music, sequences, etc) and saved the updated/older fcp project as a new file. i then closed fcp, turned on the external drive with the files on it, opened the newly created fcp project and ‘voila’ it opened with my newest sequences and other updates intact.