Sorry for the confusion. What I meant was that on many boards, the usb drive will show up under hard drives, not removable media. So it becomes part of the hard drive stack. The hard drive stack has a priority list as well. So, you have to put the flash drive with UNRAID on it to the top of the stack. Then you have the other boot section which is by device type, ie) Removable Media, cd/dvd, hard drive, etc... Under that section, select your hard drive to boot from, which due to being the top of the priorty list of hard drives, your flashdrive will be the selection. Adding/removing drives can mess with your hard drive priorty, and often you'll have to go in and adjust the bios in order to boot after changing hardware.
Hope that was less confusing, just had my first sip of coffee for the day.