How to Fix Error Code 12 in AOMEI Partition Assistant: PreOS Mode Guide

Disk management software is designed to perform complex operations—like resizing partitions or cloning disks—while keeping your data safe. However, because these operations involve modifying the fundamental structure of your hard drive, the software often clashes with the Windows operating system, which fights to keep those drives active and "locked" for its own use.

One of the most frequent results of this conflict is AOMEI Partition Assistant Error Code 12.

You typically encounter this error when attempting to apply changes, such as extending your C: drive or merging partitions. The progress bar may stop, and a message appears advising you that the operation cannot be completed in the current environment.

This error can be intimidating, but it is not a sign of data loss. It simply means AOMEI Partition Assistant needs a "quieter" environment to work—specifically, an environment where Windows isn't running in the background. In this guide, we will explain why Error Code 12 happens and provide authoritative methods to resolve it using PreOS Mode.

Understanding Error Code 12: The "Exclusive Access" Issue

To understand Error 12, you must understand how Windows protects its files. When you run Windows, thousands of background processes are reading and writing to the disk simultaneously.

If you ask AOMEI Partition Assistant to resize a partition, it needs to gain Exclusive Access (or a "Lock") on that drive. It essentially tells Windows, "Stop using this drive so I can rearrange the data."

Error Code 12 occurs when:

1. System Drive Interference: You are trying to modify the System (C:) drive, which Windows cannot fully unlock while it is running.

2. Pagefile Conflicts: Virtual memory files are locking specific sectors of the disk.

3. Background Apps: Antivirus or backup software is actively scanning the drive, preventing AOMEI from locking it.

Since the software cannot guarantee data safety while other programs are interfering, it stops the operation and issues Error 12. The solution is to move the operation to a mode where these programs aren't running: PreOS Mode.

Method 1: Restart into PreOS Mode (The Standard Fix)

This is the official solution recommended for Error Code 12. PreOS (Pre-Operating System) mode allows AOMEI Partition Assistant to run in a lightweight environment before the full Windows OS loads.

How to perform this:

1. Initiate the Operation: Set up your desired partition task (e.g., Resize/Move Partition) and click "Apply".

2. Encounter the Prompt: When Error Code 12 (or a notification about locking the volume) appears, the software will usually present a dialog box with three options:

   • Retry

   • Restart into PreOS mode

   • Cancel

3. Select PreOS: Click the option to "Restart into PreOS mode" (sometimes labeled as "Restart Now").

4. Allow the Reboot: Your computer will restart. Do not force shutdown during this process.

5. Native Mode Execution: Instead of your desktop wallpaper, you will see a basic interface (often a black screen with white text or a simplified AOMEI window). The software will automatically execute the partition resizing here.

6. Completion: Once the progress bar hits 100%, the computer will restart again and load Windows normally.

This method solves Error 12 in 90% of cases because Windows services are not active to block the drive.

Method 2: Create Bootable Media (WinPE)

If Method 1 fails—perhaps your computer hangs during the restart or refuses to enter PreOS mode automatically—you need a more robust solution. Creating WinPE (Windows Pre-installation Environment) Bootable Media is the "Pro" way to bypass Error 12.

This involves putting AOMEI Partition Assistant onto a USB stick and booting from it.

Steps:

1. Prepare a USB: Insert a blank USB flash drive into your PC.

2. Open Bootable Media Wizard: In AOMEI Partition Assistant, click on "Tools" > "Make Bootable Media".

   

3. Follow the Wizard: Select "USB Boot Device" and choose your flash drive. Click "Proceed" to burn the WinPE files to the USB.

4. Enter BIOS: Restart your computer and press the specific key (usually F2, F12, or Del) to enter BIOS/UEFI settings.

5. Change Boot Order: Set the USB drive as the primary boot device.

6. Fix the Partition: Save changes and reboot. The computer will load AOMEI Partition Assistant from the USB stick. Since your hard drive is completely offline (idle), you can resize or move partitions without triggering Error 12.

If there is still any problem, please feel free to contact us at [email protected]

Why Error 12 is a "Good" Error

It is important to remember that Error Code 12 is a safety feature. If AOMEI Partition Assistant tried to force the operation while Windows was still writing to the disk, you could suffer catastrophic data corruption or a "Blue Screen of Death."

By flagging Error 12 and forcing you to use PreOS or WinPE mode, the software ensures that your hard drive is in a stable, static state before it moves any data sectors. This guarantees that when the operation finishes, your files are exactly where they are supposed to be.

To Sum Up

AOMEI Partition Assistant Error Code 12 indicates that the software cannot gain exclusive control over the hard drive because Windows or other applications are using it.

To resolve it:

1. Use the built-in option to Restart into PreOS Mode.

2. If that fails, create a WinPE Bootable USB to perform the task offline.

3. Run chkdsk to ensure the disk is free of errors.

By moving the operation outside of the active Windows environment, you can bypass the restrictions and successfully manage your partitions