When you boot your computer, you may get an error message saying GPT formatted disk Legacy boot not supported. This error occurs sometimes after you made some changes either on software or hardware intentionally or unintentionally.
As the error suggested, your are currently using Legacy boot mode that only supports booting from an MBR disk. You may have installed a new hard drive with GPT partition table that is only allowed to boot with UEFI mode. After installing the new GPT disk, your computer may switch the first boot order to it. Therefore, you will get the GPT formatted disk error.
This error will also happen when you convert MBR disk to GPT disk without changing boot mode. .
If your computer support UEFI and Legacy BISO-based boot mode, you will also get this error when you disable UEFI mode.
Enable UEFI boot mode
If you have installed a new drive onto your computer, you can go to BIOS settings by press a specific key, usually F2 or Del, at startup. At Boot tab, make sure your original hard drive is set as the first boot device. If your computer has support for both Legacy and UEFI, you can go to BIOS and change BIOS to UEFI mode. By default, UEFI boot order has higher priority than Legacy boot order, so you can also enable both Legacy and UEFI boot mode if possible.
Convert GPT to MBR without data loss
If your computer only has support for Legacy boot mode, you can convert GPT to MBR, and then your disk can boot successfully in Legacy boot mode. Since your computer cannot load Windows, you have to create a Windows PE bootable media to do that. Most of the PE tools allows you to convert GPT to MBR, but it will also remove everything on your hard drive, which you may find not acceptable. In that case, you have to reinstall Windows to fix the error that it cannot boot from UEFI (GPT) partitions. Luckily, there is a partition manager software AOMEI Partition Assistant that allows you to convert hard drive form GPT to MBR without affecting OS. When the conversion is done, the EFI partition MSR partition will be converted to system reserved partition.
To convert disk from GPT to MBR with AOMEI Partition Assistant:
1. Download and run AOMEI Partition Assistant Professional edition on another computer.
2. In the main interface, click “Make Bootable Media”in the left panel and follow the wizard to create a bootable USB drive or CD/DVD.
3. Connect the bootable media you just created to your computer that has problem to boot, and boot from it.
4. It will bring your the same interface as you previously saw. Then right click on the disk and select “Convert to MBR Disk”.
5. Click “OK” to confirm. Then you can preview the outcome of the process. If no problem, click “Apply” apply this change to your hard drive.
After this steps, you should be able to boot your computer succefully without receiving the error GPT formatted disk legacy boot not supported. As you know an MBR disk support at most 4 primary partitions or 3 primary partition plus many logical partitions. If this GPT hard drive has more than 4 partitions, the extra primary partitions will be automatically converted to logical partitions to ensure all data are intact. If you receive this error on a Windows server, you can use AOMEI Partition Assistant Server edition.