Is it possible to boot from a USB flash drive on a Toshiba Tecra A2?

I don't know which BIOS version is yours, but did you try to change your BIOS settings? I believe you can enter in your notebook's setup by pressing F1 after it's turned on. Then check "boot devices" and see if "USB" is taking precedence from "Hard Disk" and other peripherals.

"You can override the settings and manually select a boot device bypressing one of the following keys while the computer is booting:
press ""U"" to use usb"