I must be missing something obvious. I've taken over the development of a customised Ubuntu installation, which uses the 11.04 Desktop Live ISO with a customised preseed file to make the installation of a new PC completely automatic - build the ISO with the UCK tools, burn it to a DVD, boot a PC from the DVD, go for a nice cup of tea, and come back to a finished installation just waiting for you to click to reboot into the new system.

It all works fine except for this: I cannot get a late_command in the preseed file to run. Everything else in the file works fine - partitioning, choosing packages, creating a user, etc - but the last line of the preseed file:

d-i preseed/late_command string /bin/echo "trying echo" > /var/log/myfile

...doesn't happen. I've tried every possible combination of in-target and prefixing /target/ to my paths. I've SSHed to the machine while the automated installation is running and verified that my output paths exist and are writable. I've checked the kernel command line to make sure the right preseed file is being used. My late_command command just sits there, refusing to be run. We've been sitting here staring at each other, for quite some time now. My brain has started dribbling out my ears, and I've just plain started dribbling.

Is there some crucial piece of ancient Oriental preseed-fu I'm missing? I'm at my wits' end here (and my mum will tell you I didn't have far to go) - any help will be most appreciated.