Recently I needed to boot a Dell server (PE1950) from a USB flash drive. The flash drive contained a bootable image from the CentOS 5 DVD (images/diskboot.img), to run the CentOS installer, and kickstart the machine over the network.
When cold booting these machines (eg. from a power down state), there is no problem at all. I simply hit F11 to enter the boot menu, choose the Front USB flash drive that shows up in the menu, and the machine starts booting - no problem here.
However, after completing the install, I want to boot from the USB device again, so during the reboot I hit F11 again, and what happens? No USB flash drive to be found in the menu ... At first I thought the CentOS kickstart install somehow also erased my USB flash drive, so I added several options to the kickstart file to ignore the sdb device (the USB drive), and to only use the sda device to create the partitions. So, after kickstarting the system again, I tried to boot from the USB device again, and guess what happened... Right, no USB device shows up in the boot menu.
Turns out that I need to POWEROFF the machine, then cold boot it again, and ONLY THEN it recognizes the USB device to boot from... Nice BIOS in these machines :-|