Fix Error Loading Operating System Error

Contents

1. Symptoms of the problem

When computer is powered up, an error message "Error loading operating system" appears (white-on-black letters in text mode and blinking cursor):

Boot Problem - Error Loading Operating System

2. Normal boot sequence

Normally computer boots as shown below (Windows 7 boot sequence is shown as example):

Normal boot sequence of Windows 7
  1. BIOS loads 1st sector from HDD (Master Boot Record), verifies 55 AA signature and transfers control to Master Boot Record.
  2. Master Boot Record scans partition table for active partition, then it loads 1st sector from active partition, verifies 55 AA signature, and transfers control to Boot Sector.
  3. Boot Sector loads 15 additional sectors from the beginning of disk (Volume Boot Record) and transfers control to first additional sector of VBR.
  4. Volume Boot Record loads BOOTMGR file from root directory of boot partition into the memory and transfers control to it.
  5. BOOTMGR loads Boot\BCD registry hive and displays boot menu. BOOTMGR is able to run memory test and set safe mode boot flag per explicit user request. But normally, it loads winload.exe when computer boots normally (cold startup) or winresume.exe (when computer resumes from hibernation).
  6. Winload.exe opens SYSTEM registry hive, gets list of drivers, loads NTOSKRNL.exe, loads drivers (*.SYS files) and transfers control to NTOSKRNL.exe.
  7. Winresume.exe loads memory dump from hiberfil.sys and transfers control to it.

3. Defect in the boot sequence which leads to Error Loading Operating System error

Defect in the boot sequence which is a reason for Error Loading Operating System is shown below:

Reason for Error Loading Operating System
  1. BIOS loads 1st sector from HDD (Master Boot Record), verifies 55 AA signature and transfers control to Master Boot Record.
  2. Master Boot Record scans partition table for active partition. There is one active partition, so Master Boot Record tries to load 1st sector from active partition, but sector is either outside device sector count or does not contain 55 AA signature. So MBR emits "Error Loading Operating System" error message and halts machine.
  3. The rest of usual boot events does not happen.

4. How to fix Error Loading Operating System error

1) Download EBCD and burn it to CD/DVD according to the instructions, then set up your BIOS to boot from CD. You need full EBCD version to actually write changes to the disk, but it's recommended to try demo version first to make sure  there are no hardware incompatibilities.

2) Boot from EBCD, wait until the main menu appears.
Choose "Mount & Boot Center" in the main menu or press F5 and wait while EBCD scans all local disks for Windows installations

Error Loading Operating System Fix with EBCD - Main Menu

3) Find boot loader for your primary OS in the list. Please note that "called by BIOS" radiobutton is off:

Error Loading Operating System Fix with EBCD - Bootloader is not called by BIOS

4) Click on the radio button "called by BIOS" for bootloader of your primary OS. It should be turned on now:

Error Loading Operating System Fix with EBCD - Bootloader is now called by BIOS

5) Click OK to close the window and then choose "Reboot" in the main menu. 'Error Loading Operating System' boot problem is fixed now.

Error Loading Operating System Fix with EBCD - Probem Fixed, Rebooting

Please note: Mount & Boot Center verifies entire boot path from BIOS to the OS boot loader, so other boot problems may be automatically fixed at this point.