1. Insert the flash drive into your Windows machine and back up anything from the flash drive that you wanted to keep. We are going to reformat it.
2. Open up a command prompt.
3. Type diskpart and press Enter.
After a second you should have the prompt DISKPART>
4. Type list disk. A list of your connected hard drives will appear.
Make sure you see your flash drive on the list.
In the example below my 4GB flash drive is Disk 2.
Command – DISKPART
5. Type select disk
2 but change the disk number to whatever your drive is. Make sure you get this right.
If you continue with these steps on the wrong disk you will end up erasing all of that drive of contents. Be careful.
6. Type clean. It should only take a few seconds to clean.
7. Now type create partition primary. This command should complete almost immediately.
8. Type select partition 1
9. Type active
10. Type format fs=fat32 quick to format the flash drive.
11. Lastly, type assign and then exit
12. Format complete! You can close the command prompt.
After finish format you can use ultraISO to extract out the Bootable ISO to the thumb drive.
Use this method i have created a lot of bootable thumb drive
Bootable SCCM ISO.
Bootable Windows 7 / 8
Bootable Acronis ISO
Whatever bootable ISO you also can use method to boot. Enjoy carry bootable OS Thumb drive.