Thanks farslayer (whoever you are).
I am not sure how to build the repository, and the machine I am causing the problem is not connected to anything else. Given the info you gave me (thanks again for that), my copy to usb script now looks like
#!/bin/sh
#
cd /var/cache/apt/archives
cp -puv *.deb /mnt/usb/archives
dpkg-scanpackages . /dev/null | gzip -9c > /mnt/usb/archives/Packages.gz
sync
and I added a line to the bottom of /etc/apt/sources.list (on the computer that I want to maintain) that looks like
deb file:///mnt/usb/archives ./
and everything seems to work fine
Thanks again,
Steven Peckham