There are 3 primary conditions that must be met on a ThumbDrive to enable bootability.
a. Master Boot Record (MBR)
b. Active Partition
c. System Files
Aside from enabling support on ThumbDrive side, the host side (Computer, notebook with USB
port) must satisfy the following condition
a. BIOS that supports booting from external USB mass storage device. E.g. Phoenix BIOS
version 4 release 6.
Setting up the BIOS
On some BIOS, the boot order will directly affect whether a device is seen by fdisk. To be
sure, plug in the ThumbDrive to the USB port, boot up your computer and access the BIOS to
change the boot order of the computer such that the ThumbDrive is checked before the hard
disk. After rebooting, run fdisk to create the primary partition on the ThumbDrive (refer to
Setting up the ThumbDrive)
Setting up the ThumbDrive
1. Creating a MBR on the ThumbDrive
Boot a computer to DOS environment with a DOS boot disk. Run ?fdisk? and create a partition
for the ThumbDrive. Be sure to confirm the target disk within fdisk (option 5) so that you do
not accidentally change the partition of the computer hard disk. If you do not see ?option 5? or
the ThumbDrive available within fdisk, refer to Setting up the BIOS to enable the support.
2. Set the partition Active.
Within ?fdisk? environment set the newly created partition to active.
3. Transferring System Files
Under DOS environment, run ?sys drive letter:? to transfer the system files to the ThumbDrive.
Alternatively, run ?format drive letter: /s? to format and transfer the system files to the
ThumbDrive.
Under Windows 98 environment, run format utility and select ?system files? under format
option.
The system files cannot be transferred within Windows ME, Windows 2000 and Windows XP
environment.
(taken from here:
https://www.experts-exchange.com/Operating_Systems/Q_20872552.html)