How to UN-Partition and External Hard Drive

As I suspected this external hard drive is a RAID drive which means it has two physical hard drive installed internally and configured as a RAID0 stripe which allows the two separate 2TB drives to act as a single 4TB drive.

The WD support site is terrible since there are not download links from the product page for your G-Raid. I did a quick search and found this page which has a link to the G-RAID Software Utility for macOS, unfortunately I don't know if this software is compatible with your G-RAID (again due to poor documentation on WD's site). You will need this software or its equivalent to reconfigure your G-RAID into a RAID0 mirror if you want the G-RAID device to appear as a single 4TB drive. Doing this will destroy all data on both of those 2TB Hitachi drives, so make sure you have transferred the data on them to a safe location until you reconfigure the G-RAID.

https://support-en.g-technology.com/app/answers/detail/a_id/50647

Here is the general G-RAID support site taken from the URL of the above link....could not find it with Google or WD searches. You can select your exact model to look for support. I don't see any download options for software, unless the site is not working correctly in my browser, but at least this should point you in the right direction.

https://support-en.g-technology.com/app/products/detailnew/p/15/~/15

FYI, you should always have frequent and regular backups of any external media which contains important & unique data.

Copy everything on the WD drive off to another drive.

Go to the View box in the upper left corner and select Show All.

Select the WD drives first Icon in the list of drives and select the Partition tab.

Now see if you can add the second Partition back to the first Partition using the pie wheel.

If not select the entire WD drive, then erase and reformat the entire WD drive back to one partition.

The opposite of > Partition a physical disk using Disk Utility on Mac - Apple Support

If you cannot get them to help you & their software doesn't work, then you can use Disk Utility to create a Software RAID0 stripe to combine those two drives into a single 4TB volume.

Create a disk set using Disk Utility on Mac - Apple Support

Before you do anything, go into Terminal and enter the following command. It will provide much more detail than Disk Utility does:

diskutil list

Post a copy of the results. (All it does is provide details about each of your disks; there is no personal information.)

What is the exact make & model of this external drive? Or if you built this yourself, then what is the exact make & model of the drive enclosure being used?

If these two physical Hitachi 2TB drives are contained within a single device enclosure, then that device enclosure may have built-in hardware RAID options which would allow you to stripe these two separate physical hard drives together into a single 4TB entity. However, this is very risky since you will lose all data if either one of these two physical hard drives fails or has any problems.

From what I'm seeing in the screenshot of Disk Utility, it appears you have two physically separate 2TB Hitachi hard drives. Usually the make & model of the enclosure, drive dock, or adapter tends to appear instead of the model of the physical hard drive itself.

We are definitely missing critical information about this drive and your setup/configuration.

Have you tried selecting the Hitachi drive at this point and erasing?

If you have any fils on the drive they should be copied off to another volume before erasure.

Sorry my bad... I referred to the drive as being a WD, when it is a Hitachi.