If you set the "boot sequence" so that "external drives" are checked for a bootable load before C:\ is checked, the computer should boot from the external optical drive if the disk is in it. If it doesn't find something bootable there, it will go on to the internal drives and will boot from C:\.

(My laptop flashes F2 for setup and F12 for boot sequence, but there seems to be some variation in that.)

It's likely that the boot sequence already will be set to look for other bootable devices before C:\ without any changes, but I don't really know if that's the case with the semi-computer ittybitty machines. As already mentioned, if only one OS is installed on the computer, the default setup should boot from any bootable external device without the necessity of a boot manager. If otherwise, you wouldn't be able to install a new OS if the old one craps out. The boot manager is only necessary if you have more than one OS on the internal computer drives so that you have to choose between them each time you boot up.

The download of account information should be from a secure site, so as long as the "secure OS" on the "live CD" is the only thing running it should arrive safely on your machine regardless of where you save it, and a fresh USB stick is about as good as anything. There have been reports of "new USB drives" carrying malware, but all that I've seen reported have been extremely isolated cases, on a par with the few software manufacturers who've sold program installation disks that inadvertently were infected. Both are extremely rare.

If you're really worried, you might boot to your safe OS and reformat the "stick" using your "safe OS" before putting anything extremely sensitive on it, but that's probably overkill.

It might be noted that some external optical drives like more power than is available from a single USB connection, so some of them provide a "pigtail cable" so that you can plug them into two separate USB ports on your computer. The specs you see when you're ordering one seldom make this clear.

Since your computer only has three ports(?) a forked-tailed disk drive and one USB memory stick would put you at APF (all ports full) but you can work around that with a powered external USB hub. Plug the hub into one computer USB slot, plug in the wall wart (transformer) in a line socket, and have more ports with enough power to run what you plug into them. If you use a passive hub the computer still has to provide the power for everything you plug into the hub from the single USB port you plug the hub into, so even though you might have "more ports" there's no more power to run what you plug into them, so there is a small risk of overloading the computer's USB port.

Of course since you got the tiny computer for portabiiity you may not want to carry a bunch of accessories along when you're out rambling, but at least at home a decent powered USB hub can come in handy. (like if you aren't wireless for your printer connection and want to print something, or you need to dump the flash card from your camera or ... and ...?)

John