Whenever I try to do this, the construction set immediately hangs, adds 15 megabytes to the size of the file, and promptly corrupts it.

Is there something I'm doing wrong? : /

No, that's a known problem. Use Wrye Bash or the http://www.gamesas.com/index.php?/topic/1198521-rel-construction-set-extender/ to do it.

Well, it gave me a hint as to what was going on. Apparently I have to run the construction set as administrator even though the game is on my J: drive.

Thank you.