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.
Open an elevated command prompt and type diskpart. If the disk does not contain any partitions or volumes, skip to step 6.
At the DISKPART prompt (right-click Command Prompt, and then click Run as Administrator), type list disk. Make note of the disk number you want to delete.
At the DISKPART prompt, type select disk
At the DISKPART prompt, type clean.
Important: Running the clean command will delete all partitions or volumes on the disk.
At the DISKPART prompt, type convert mbr.
list disk: Displays a list of disks and information about them, such as their size, amount of available free space, whether the disk is a basic or dynamic disk, and whether the disk uses the master boot record (MBR) or GUID partition table (GPT) partition style. The disk marked with an asterisk (*) has focus.
select disk: Selects the specified disk, where disknumber is the disk number, and gives it focus.
clean: Removes all partitions or volumes from the disk with focus.
convert mbr: Converts an empty basic disk with the GUID Partition Table (GPT) partition style to a basic disk with the master boot record (MBR) partition style.