Universal-USB-Installer.exe from http://www.pendrivelinux.com is great tool to make any linux boot on USB flash drive, but one weird thing happened to me, I bought couple 4Gb USB flash, none of them work and giving this Syslinux load error :
No Default or UI Configuration Directive found!...
but when I use my smaller flash drive (ie 1Gb and 512Mb) it work just fine.
figured out for a while, typed this on the prompt below that error message :
grub# vmlinuz initrd=initrd.gz pmedia=cd
with that line typed after the error, my usb boot work just fine, then I checked what files are inside my flash drive, I found there is no syslinux.cfg, only isolinux.cfg, so I renamed the file to syslinux.cfg, and bam, every time I rebooted the machine, it work without any intervention.
I know since long time ago, isolinux.cfg is for CD boot, and for HDD boot, it use syslinux.cfg, but not sure why my 1Gb and 512Mb flash drive able to boot its linux kernel with isolinux.cfg.
anyway, just change isolinux.cfg to syslinux.cfg, and it will take care the problem.