you appear to need to copy the entire tftpboot directory to the root of the flash drive, not just the fog directory under tftpboot.
you may want to adjust the steps involved in this..
Issue booting newer Dell's using Syslinux
I am having issues getting Dell 990s to boot using the Syslinux method. Upon booting the screen just goes grey. Any ideas? I've tried converting the menu to use DEFAULT menu.c32, but couldn't get that to do anything but display the syslinux version line. Thanks
The case of the missing tftpboot contents
As a None Linux user, the latest download of fog itself does not appear to contain a working tftpboot folder. There is a \packages\tftp\tftpboot folder, but it is empty, and while there are fog and pxelinux.cfg folders in the tftp folder, following the instructions on this page results in a "boot failed" error. I for one would appreciate the Linux egg sucking instructions for us simpleton Windows users.
Keep up the great work