This is the first time I encounter this issue. I can't boot from USB drive on a Sony Vaio PCG-21313M. I set the external device as first choice and enabled it. Also I tested the usb (Windows 7) on other laptops and it boots fine. The computer doesn't boot properly not even in safe mode.
If your drive is bootable then follow this step.
- Go to bios by pressing "F2" on power on, to enter bios.
- On boot priority section enable "external boot".
- Set "external" as first priority and "internal hdd" as last. by pressing "F5/F6"
I am sure it will work, Make sure "internal hdd" is at last priority, because if it is not sometime it did not boot from "eternal" although it is set as first priority. unknown result.
If it did not work possibly you may problem with your external drive or data(boot-sector).