I forgot to add in the original post: the situation I said I found myself in does not normally have access to the internet. Rural, developing country, I would not be allowed to make any changes to the computer, and so on. So booting from the network would not be an option. Nor is reconfiguring/updating the bios, even if I know how.
I am also looking at Slitaz, one of the option suggested by jefro. However, I am somewhat skeptical. After a lot of googling, I learned that there are a number of unix distros out there that will bypass the limitations of the bios and boot an unix os from a usb hd. Unfortunately they will only boot the same distro, ie slax on cd will only boot slax on usb. puppylinux or dsl, on cd, will boot only their respective distros on usb. Plop seems to be the only one that will do this with whatever distro. On the other hand that might also explain why plop tends to be a hit or miss affair. There are a lot of posts out there saying that plop did not work for them.
I am using the latest version of plop, by the way. I have also used plopkexec. Neither worked for me. My suspicion is that it might be because of my particular ext usb hd. The plopkexec booted successfully a knoppix installed on an 8 Gig flash drive, but not the xubuntu on the 60 gig usb 2.0 ext hd. But I am just speculating -- I am not much of a tech head, and this is all a bit above my head.
I think I would like to go with this solution: the "limited Linux on cd" from the Ubuntu Community Documentation
. But I'd appreciate very much any help in modifying this solution to use GRUB2 rather than GRUB.
Thanks again everyone.