Changed MBR and Now Boot Device Not Found, What to Do?

In the process of installing Windows 11 on my HP laptop, I was asked to change MBR to GPT on my laptop. I put in mbr2gpt/convert in the command prompt, and since then, every time I start my laptop, it shows 'Boot Device not found' with a 3F0 error. What should I do?

According to your description, the laptop shows "Boot Device not found" every time you start it since you convert the MBR to GPT while installing newer Windows. Most likely because you just converted the partition style from MBR to GPT but still need to change the boot mode to UEFI. Usually, the newer OS devices support UEFI, while the old Windows use Legacy BIOS. So, in this case, changing the boot mode from Legacy to UEFI is a must-done job.

Here is full detail about how to enable UEFI mode:

There is another possibility that you have lost some data during the conversion process, which causes your computer can't find the boot device. To make things easier, you can use reliable third-party software - EaseUS Partition Master, which provides Partition Recovery Wizard to restore deleted or lost partitions.

Download this tool and try to recover the lost data with easy steps:

Since converting disk from MBR to GPT via Windows built-in utility will delete data and partitions, you'd better use EaseUS Partition Master to complete the conversion. Find more information in the below link:

More Information: 4 Ways to Convert MBR to GPT