You can convert your GPT disk into a traditional MBR disk by writing zeros in order to destroy the partition table and initialize the hard drive from scratch. External booting is not supported by Western Digital though, which means even after properly formatting your unit there’s no guarantee you’ll be able to boot from it.
As for the file system, you can format it in Windows Explorer and Disk Management from exFAT to NTFS as a regular hard drive even if you don’t change the partition table from GPT to MBR.