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#21
What did you use to create the partitions on the usb drive?
Did you use win7 diskpart.exe?
Have you set a partition Active?
Boot indicator bytes in partition table should be either zero or 0x80 (that means bootable).
The message indicates those values are not right.
You might try using a free 3rd party partition manager.
e.g. http://www.partitionwizard.com/
Have a look at the usb drive.
Inactivate and reactivate the install partition on the usb drive.
See if that fixes it.