Booting ubuntu from a flashdrive for a computer that does not support usb booting:

[struggle4progress]

The underlying idea is that since one can boot from a cd, one can prepare a modified boot cd that begins the boot process then switches control to a flash drive; instructions for preparing the helper cd can be found here ('Booting the kernel from a bootable CD'):

BootFromUSB
https://help.ubuntu.com/community/BootFromUSB

I tested these instructions as follows. I prepared the iso file for burning the cd, then handed that iso to a virtualbox machine. I next prepared a thumbdrive installation of mint using unetbootin with a mint iso. Finally, I attached the thumbdrive to the virtualbox machine while it was booting from the helper cd iso, and the result was a successful boot of mint. This is a credible test because virtualbox does not directly support a usb boot on my osx host platform. For a second test, I burned a cd from the iso and used an older machine that has usb ports (and a bios that supports cd boot but not usb boot), attaching the usb and booting from the cd; the boot was somewhat slow but successful