Put the drive into a working Win 8 or Win 10 PC.
Goto the Disk Management console.
Note down the drive number of the disk you want to convert to MBR.
Now run the COMMAND PROMPT with admin permissions
Type: SELECT DISK (drive number of the drive you got from the disk management console). Example: SELECT DISK 2
Be very careful that you select the correct drive, as the above cannot be undone.
Now go back to the disk management console and create the partition type on the cleaned drive to be MBR