UPDATE: See the comments below for additional help regarding the issue.
I created a bootable USB pen drive with Ubuntu 10.10 Maverick Meerkat Alpha Daily Build on it using Startup Disk Creator (usb-creator-gtk) in Ubuntu 10.04 Lucid Lynx. This gave the following message during startup:
SYSLINUX 3.63 Debian-2008-07-15 EBIOS Copyright (c) 1994-2008 H. Peter Anvin
Unknown keyword in configuration file
boot:
It seemed the version of syslinux in Lucid has a bug. I removed syslinux via Synaptic (which also removes usb-creator-gtk and usb-creator-common) and downloaded the deb files for the latest version of syslinux and syslinux-common from here and installed them. Then I installed usb-creator-gtk and then built the Live USB again. Then, it booted fine without problems.
