Export crashes Fcp and Finder and empties Start-Up Disk

I am editing a 10-min Apple ProRes ,1920x1080, 23.98fps sequence (footage from 7D normal speed and slowmo conformed with Cinema tools). I work on an external Lacie 2BIG.
Everything works fine in editing and rendering. The problem is in export. When it finishes exporting the sequence, it crashes fcp, I am even unable to successfully force quit it, and it also crashes the Finder. I reboot the computer leaving the HD on. When I try to open the exported sequence. mov file, it crashes Finder. On top of all that, I cannot eject the HD from the computer. In the background, in the Activity Monitor, I see that my startup disk from 17gbs, within half an hour it reaches to 0kb and an empty start-up disk message appears. Only then, I can eject the disk. When I restart the computer, without the disk, the start-up disk memory is back to normal.

The result is I cannot successfully export the sequence.I tried the process with a different HD and the same happens. The problem seems to be only with that project.

Please let me know what is going wrong, in five years time, it's the first time I encounter this issue.

MacBookPro with 2.2Intel Core Duo, 2GB Ram, MacOSX 10.5.8, Final Cut Pro 6.0.5

Thank you all, in advance!

Try manually deleting all render files in the Finder first.
Use the free Digital Rebellion app "Preference Manger" to trash pref files.

Thank you Ben!

I already manually deleted render files, but it didn't help. I will now trash preferences and see what happens.

Ben, I have trashed preferences, but the problem did not go away. Now, I cannot even edit inside and the same crash and start-up disk been eaten up, happens. Any other ideas?

You can not eject drives while FCP X is launched.
No clue what "eaten up" means.
No clue where you see your boot drive being 0kb, but if that were true, you couldn't run your computer.

Get Event Manger X and move the bad project to be hidden. See if things run properly then. If so, you have a severely corrupted Project database. You can do it manually, but Event Manager X is cheap and is super handy to have around.

Event Manager X won't work because I have FCP 6.0.5
I can see my boot disk getting empty in Activity Monitor in Disk Usage. And that happens even when I have rebooted the disk and FCP is shut down, but the external disk is still on.

Activity monitor doesn't show your drive getting "empty", it shows a decrease in the system accessing it. "Empty" means there is no data on it. Low usage means the system is simply not accessing it as much, which may mean nothing.

You may have a corrupted project file. If you've deleted all render files, and launching that project still does that, there is either very corrupted media, or the project file itself is corrupted and lost.

Personally, I'd run Disk Warrior on all my drives to be safe.

Try this. Make sure the Project file is on the external drive. Disconnect the external drive. Boot your Mac. What happens when that troublesome project is gone?