Booting USB Thumbdrives

I have been trying to work on making a usb thumbdrive bootable into DOS. I’ve been to a few websites where they’ve given step by step instructions as to how to do it. I managed to get it to be semi-bootable after trial and error with a couple utilities.

Even setting usb to boot first before hard drive I couldn’t get it to set up the thumbdrive as the primary, so i disconnected the power cable from the hard drive. I’ve gotten it to boot by using fdisk and format /s commands.

My problem is that I can’t get it to boot to usb as long as an internal hdd is connected. As well so far it only seems to work in the computer I created it on and no others.

Does anyone have any ideas?