This problem is usually do to the missing "active" flag, but you've already addressed that.
Try booting the flash drive on another machine. If it isn't a Lenovo (and maybe if it is, but isn't an e220s) it won't boot all the way up. It will probably start R&R, then reboot. The point here is to verify that it's bootable at all.
If it isn't, the problem is with the flash dive or how it was made bootable. Another thing you might try is to spin up a Linux live CD and use fdisk to verify the active setting of the flash drive's partition.
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