Bootable flag on USB drive?

I confess to being a bit frustrated here. I've made a bootable USB flash drive 
according to these instructions:
http://www.pendrivelinux.com/ubuntu-810-install-using-the-built-in-usb-installer/
but it won't boot. It doesn't even try to boot. Another bootable drive in the 
same computer boots OK (puppy linux).

I don't know how to debug the existence of an MBR or a bootable flag on this 
drive, to identify what is wrong.

The windows tools that I use for CD's that will handle boot records (e.g. 
magicISO) don't seem to work on flash drives.

The windows disk management tool (diskmgmt.msc) doesn't even mention bootable.

The Ubuntu disk tool doesn't mention boot sectors or bootable flags either.

I recall that I can at least see the bootable flag in fdisk or cfdisk, but 
neither will work on the flash drive, as far as I can tell. If I say "cfdisk 
/dev/sda" it gives me an error, and if I eject the drive but leave it physically 
plugged in, it still gives me an error.

Does an ISO file include the boot record? It seems that if I use one of the 
windows tools to extract an image, it's not included; but if I deliberately suck 
the boot record off the CD (in this case) then I can store it in the ISO. Is 
that because the boot code is dependent on the medium, and the ISO should be 
medium independent?

Does a USB flash drive even have a bootable flag?

Are there any key words that I should be googling for (e.g. go look up 
'syslinux')? What tools (Ubuntu or XP) could I be using to view/check this stuff?

Any basic tutorial on boot sequence would be handy; I think the BIOS checks for 
the bootable flag on the drives listed in the boot order, then loads and 
executes code in a specific location (MBR?) which is not a file, per se. MBR 
code loads and runs a specific file, or possibly reads some config file and 
loads whatever's listed there. Am I right?

Hints welcome.

Thanks

-- 
Andy
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