I own a Thinkpad X121e and I upgraded its Ubuntu from 13.04 64bit to 13.10 64bit using the Update Manager. When the upgrade process suggested to restart the system, I restarted the computer and now it says "Operating system not found" after a very brief splash screen.
The splash screen says something like (too brief to see everything):
Intel UNDI, PXE-2.0 ... For Atheros ethernet Controller ... Check cable connection! PXE-MOF: Exiting INTEL PXE ROM
I prepared a startup USB disk using Ubuntu 13.10. I tried the startup USB and it works very well on other computers. So I changed the booting order of X121e and tried all three USB slots, all said "Operating System not found" without any splash screen.
I also tried using startup USB with 12.04 LTS. Same results.
I do not really want to reinstall the system now. Any hints on how to proceed?
ps. I have tested the memory and hard drive using Lenovo software and both passed the test.
Problem solved: 1. It turned out the USB was formatted in NTFS by default on a windows machine. Changing it to FAT32 makes the laptop recognizes it; 2. After booting from the USB, I installed and ran boot-repair and the system is fixed.