Is it possible to pass on the OS you're on? If you're on Microsoft's Windows OS, you can use Diskpart to change your HDD back to a Basic Disk but your data will end up being erased. You will
have to rely on a third party tool like EASEUS to deal with your drive with data while making progress to a Basic Disk.
About Diskpart,
Open CMD in an elevated command(
right click>Run as Administrator)
Type
diskpart
Tap
Enter
Type
list disk
Tap
Enter
Type
select disk x(the disk listed as your 3TB HDD which is denoted by a number) - if you have only one drive connected then you're only going to see Disk 0.
Type
convert basis
Tap
Enter
You should see a message stating that your HDD is now converted to a basic drive.
Might want to
follow through this video.