How To Change GUID Partition Table to MBR

To change a GUID partition table disk into a master boot record disk using command line, follow the steps below.

1. Back up or move all volumes on the basic GUID partition table (GPT) disk you want to convert into a master boot record (MBR) disk.

2. Open an elevated command prompt (right-click Command Prompt, and then click Run as Administrator) and type diskpart.
If the disk does not contain any partitions or volumes, skip to step 6.

3. At the DISKPART prompt, type list disk. Make note of the disk number you want to delete

4. At the DISKPART prompt, type select disk <disknumber>

5. At the DISKPART prompt, type clean

6. At the DISKPART prompt, type convert mbr

And there you have it.