The critical part is what size does Disk Utility report for the physical drives? By default Disk Utility hides the physical drives from view so you may need to click "View" within Disk Utility and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility. You would want to erase the whole physical drive. See the following Apple article (I'm providing a link to an older version through the Internet Archive because it also include a useful picture with the instructions):
https://web.archive.org/web/20250909095655/https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac
Also, older drive docks/adapters/enclosures had a 2TB limit so when a larger drive is connected, they will report a smaller size drive.
The best way to deal with a former RAID drive is to write zeroes to the beginning of the drive to destroy the partition table & hidden areas, or use a Windows PC's "Disk Management" to "clear" the drive to reset it (not sure if that "clear" option will work with a Hard Drive though, but deleting all partitions may do the trick depending on what WD does to those RAID drives).
Unfortunately for the "zero" option it requires using the Terminal command line which is a bit involved providing instructions & also risky since there are no safety nets so you may end up destroying a different connected drive. I can provide instructions if you wish.
Before doing anything it is probably best to check the health of those drives especially if you believe one of them is bad....chances are the other one may be close to wearing out as well even if it is still healthy. You can use the third party app DriveDx (free trial period) and posting the complete text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. You will need to install a special USB driver to attempt to access the health information on the USB connected drives.....some adapters/docks/enclosures may not allow the necessary communication.
