If you're on Windows PC:
1. Open Command Prompt
2. Type " diskpart "
3. Type " list disk "
Now you can see plugged disks in your PC... you should recognize your disk from Total Size
Anyway once you've recognized it on the last column (GPT) you'll see an asterisc if the disk is GPT, nothing otherwise
If your disk is GPT (so there's the asterisc in GPT column) and you want to convert it to MBR follow up with:
4. Type " select disk #here goes the disk number printed above# "
5. WARNING this command will erase your disk data so, if you have important data there, backup it before typing " convert mbr "


