a 4To external hard drive which appears in two 2To external hard drives

That’s a two-terabyte (probably usually) RAID-1 device, known as the “WD My Book Thunderbolt Duo 4TB External Dual Hard Drive Storage with RAID”; the WD WDBUSK0040JSL-NESN.

The manual for that storage hardware is here: https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/wd/product/external-storage/my_book/my-book-duo-2014/user-manual-my-book-duo.pdf

Page 60 of that document has the configuration options: RAID-0 striping, RAID-1 mirroring, or JBOD.

As mentioned above, I’d assume the usual application was RAID-1 mirroring. But it can run in any of three modes, depending on local need.

Details on erasing and initializing the storage for macOS is also included in that document.

[This is a Direct Download Link]

davacorp wrote:

What could I do to join the two internal physical drives into a single logical one (as it was before I created two RAID partitions)?

IMHO it is not a good idea to join 2 disks into a Raid0 "striped" setup, which is what your WD Passport Pro originally was. If one of the drives fails you lose the data on both drives.

But if you feel you must do that, and WD does not have an app or other means to re-set the drives to their original combined state, then reformat each drive as Mac OS Extended Journaled and then use Disk Utility to create a Striped Raid0 Set. See Create a disk set using Disk Utility on Mac - Apple Support for details.

Format the whole drive to 1 Partition.

In Disk Utility>View, select Show all Devices, highlight the top left entry for that drive.

You mention RAID, could you actually have 2 WD drives?

Format Two Drives as One

To format two drives as one on macOS, you can use the Disk Utility application to merge the partitions. Here’s how you can do it:

1. Open Disk Utility, which you can find in the Applications > Utilities folder.
2. Select the drive itself, not the individual volumes, and click on the Partition button at the top.
3. A new pane will open showing the drive as a pie chart with the partitions displayed in a ‘clock face’ style. The first partition will be on the right, from 12:00 to 6:00, and the second partition from 6:00 to 12:00.
4. Select the second partition and click the [-] minus button underneath, then click Apply.
5. After confirmation and the process is completed, select the first partition and use the handles at the edge of the pie-slice to enlarge it to the full size.
6. Click Apply and confirm.
7. Once complete, reboot the machine and use your new full-size partition.

You would either need to use the WD proprietary software to possibly make a hardware based RAID 0 Stripe, or you can use Disk Utility RAID Assistant (look on the Disk Utility "File" menu) to create the RAID 0 Stripe. I don't know how those WD dual bay enclosures work, so you may only be able to go with the software RAID of Disk Utility. Having Disk Utility managing the RAID at least doesn't involve installing any extra proprietary third party software that can possibly cause problems.

Keep in mind that if either drive fails in a RAID 0 Stripe, then you lose all the data on that RAID 0 Stripe.

Can't you just do a factory reset to return the disc to it's factory state and then start from scratch.

The manual also refers to a management interface - can you use this to merge or re-format it?

Exactly what model WD unit is this (model # please). It may in fact have two 2TB drives inside.

The "WD20NPVX" is a 2TB drive. You appear to have two of them.

Different model number, probably different enclosure, possibly different UI, same RAID-0, RAID-1, JBOD options.

The model you’ve specified seems to be related to the WD 4TB My Passport Pro Portable RAID Storage (or maybe the Elite model), as search results for that are mixed together.

All of what shows seems to be RAID-0, RAID-1, JBOD, though.

https://www.bhphotovideo.com/lit_files/104440.pdf