“An Operating System Wasn’t Found” Windows 10 – How to Fix It?

The Error: “An operating system wasn’t found” in Windows 10

The following error “An operating system wasn’t found. Try disconnecting any drives that don’t contain an operating system. Press Ctrl+Alt+Del to restart” could appear on Windows 10 during OS startup. This error can be displayed after upgrading Windows, for example, from Windows 8 to Windows 10, updating BIOS or cloning a hard drive to another. No matter what triggers this error, here you’ll find some useful methods to fix the “OS not found Windows 10” issue.

How to Fix “An operating system wasn’t found” in Windows 10?

In order to fix the issue “An OS wasn’t found Windows 10”, you must have a installation USB media. If you don’t own one, you can use MediaCreationTool to create the installation media.

Before proceeding to the following fixes, you can first try:
1. Setting your internal hard disk with the installed OS as the first boot devices in the BIOS Setup settings.
2. Disconnecting any other devices that don’t contain your OS, for example, your mouse, keyboards, etc.

Method 1. Perform a Windows Startup Repair

Windows Startup Repair can fix many issues related to Windows startup failure. So, you might as well first try this method.

1. Connect the Windows USB installation media to the PC and boot from it.

2. In the first setup window, select a language, time and currency format, and keyboard or input method. After it’s done, click on “Next” to continue.

3. In the second window, click on “Repair your computer”.

4. Then, click on “Troubleshoot” > “Advanced options” > “Startup Repair”.

Wait a few seconds before it’s completed. When the Startup Repair is completed, try booting into Windows again.

Method 2. Configure and repair the boot files

If the above method doesn’t help fix “an operating system wasn’t found” in Windows 10, you can consider configuring and repairing the boot files.

1. Connect the Windows USB installation media to your PC and boot from it.

2. In the first setup window, select the language to install, time and currency format, and keyboard or input method, then hit “Next” like what you’ve done in Method 1.

3. In the second window, press “SHIFT + F10” to bring up Command Prompt.

4. In the window of Command Prompt, type “diskpart” and hit “ENTER” to bring up the interface of DiskPart.

5. Then, input “list disk” to see whether your disk is in GPT or MBR and hit “ENTER”.

6. Locate the GPT column and check if it contains an asterisk “*”.

- If you see an asterisk, that means your disk is in GPT.

- If you don’t see an asterisk, it means your disk is in MBR.

7. Input “exit” and hit “ENTER”.

If the partition is in MBR, then, go to part 1 to find the solution; if the partition is in GPT, read part 2 to solve the problem.

Part 1. For MBR system drive

The first thing you need to do is to find on which hard drive the OS is installed.

1. In Command Prompt, type “dir c:” and hit “ENTER”. If the Windows folder is displayed, then, continue to the next step, otherwise, proceed by typing the next alphabet letter until you find which partition has the Windows folder (e.g. dir d:, dir: e, dir: f…). (In this example, the D drive contains the Windows folder).

2. Type “bcdboot D:\Windows /S D:” in Command Prompt, then, you should see this message “Boot files successfully created”.

3. After the boot file creation, bring up the window of DiskPart again by typing “diskpart”.

4. In DiskPart, type “list disk”.

5. Type “select disk x”. “x” represents the number of the disk that contains the OS – e.g. “0”.

6. Type “list volume” and hit “ENTER”.

7. Type “select volume x”. “x” represents the number of the partition containing the Windows folder. In this example, it’s “3”.

8. Type “active” and hit “ENTER”.

9. Type “exit” and hit “ENTER” to quit DiskPart.

10. Shut all windows down and reboot the PC.

11. Remove the Windows 10 bootable media and boot into Windows 10 as usual.

Part 2. For GPT system drive

1. Find out on which disk Windows lies in the method mentioned at the beginning of Part 1.

2. Type “diskpart” and hit “ENTER” to bring up DiskPart utility.

3. In DiskPart prompt, type “list disk” and hit “ENTER”.

4. Now, type “select disk x” and hit “ENTER”. “x” means the number of the disk containing the OS – e.g. “0”).

5. Type “list partition” and hit “ENTER”.

6. Check which partition is labeled as “System”. In this example, it’s partition 4.

7. Type “select partition x” to select the system partition. “x” represents the number of the partition labeled as “System”. e.g. “4”.

8. Assign a drive letter (for example, “s”) to the system partition by typing “assign letter=S:” and hit “ENTER”.

9. Type “exit” and hit “ENTER” to quit the window of DiskPart.

10. Then, type “cd /d S:\EFI\Microsoft\Boot\” and hit “ENTER”.

11. Type “ren BCD BCD.bak” and hit “ENTER”.

12. Type “bcdboot C:\Windows /l en-us /s S: /f UEFI” and hit “ENTER”.

13. Shut all windows down and restart your PC.

14. Remove the Windows 10 bootable media and boot into Windows as usual.

Method 3. Rebuild MBR with Third-party freeware

If you think Method 2 is too complex and are still looking for an alternative way to fix “An operating system wasn't found” windows 10 error, maybe it’s time to try some third-party software. With AOMEI Partition Assistant Standard, powerful and simple disk management software, you’re able to rebuild MBR to fix such Windows boot error once for all. Apart from this useful function, you can also try its other features to fix corrupted SSD, black screen boot error, or make conversion between GPT and MBR.

Before getting started, download this software first and then, make a bootable media using on another normal PC, and boot the PC from this media.

Note: If any data loss happens during your operation, you can use the "Recover Data" function to retrieve lost data.

Step 1. After getting to the main interface of AOMEI Partition Assistant Standard appears, select the system hard disk, right-click on it, and select “Rebuild MBR”.

Step 2. Set the type of MBR for the current OS and hit “OK”

Step 3. After returning to the main interface, click on “Apply” on the upper left and hit “Proceed” to start the MBR rebuild.

You’re supposed to wait only a few seconds before it’s completed.

Conclusion

An operating system wasn't found windows 10 Dell or other brands of PCs? After going through the entire tutorial, I believe you’ve found the best way to fix it. As for me, I prefer method 3 because it’s easier and really helped me out. To use other features of AOMEI Partition Assistant Standard including free space allocation from one partition to another, partition split, and dynamic to basic conversion, you’re supposed to upgrade it to the pro version.