Solution 1: Convert GPT disk to MBR disk with EaseUS Partition Master
EaseUS Partition Master can only convert empty disk without partitions. Please delete all the partitions on the disk. We suggest you back up the disk before converting by the built-in Copy Wizard of EaseUS Partition Master. Please read this article for the detailed steps: Disk Copy Wizard in EaseUS Partition Master to protect data.
Solution 2: Convert GPT disk to MBR disk with Windows Disk Management