APFS External Disk becomes Read-Only on Catalina

If Disk Utility cannot repair the APFS volume, then you will need to copy the contents to another drive so you can erase your external "WD My Passport...." drive. It appears macOS is mounting the external drive as read only since the APFS file system is damaged. Unfortunately there are currently no third party utilities capable of repair an APFS volume since Apple has not released the necessary information yet.

Erase the "WD My Passport...." drive as GUID partition MacOS Extended (Journaled). Make sure to erase the physical drive which will require you to click on "View" within Disk Utility and selecting "Show All Devices" so that the physical drive shows up in the left pane of Disk Utility so you can select it. The MacOS Extended file system is an older and much more stable filesystem than is the new APFS filesystem.

After erasing the "WD My Passport" drive, you can transfer your files back to it.

HWTech wrote:

If Disk Utility cannot repair the APFS volume, then you will need to copy the contents to another drive so you can erase your external "WD My Passport...." drive. It appears macOS is mounting the external drive as read only since the APFS file system is damaged. Unfortunately there are currently no third party utilities capable of repair an APFS volume since Apple has not released the necessary information yet.

Erase the "WD My Passport...." drive as GUID partition MacOS Extended (Journaled). Make sure to erase the physical drive which will require you to click on "View" within Disk Utility and selecting "Show All Devices" so that the physical drive shows up in the left pane of Disk Utility so you can select it. The MacOS Extended file system is an older and much more stable filesystem than is the new APFS filesystem.

After erasing the "WD My Passport" drive, you can transfer your files back to it.

The OP says "I have tried to connect the disk to a MacBook Air (High Sierra 10.13.6) and it shows the disk works well on that system." This suggests that the data is accessible, which is a good thing. And following your advice to take a copy of the data on the drive and to reformat it will likely remove the problem, and can be done on that machine, too. But it seems odd that one machine shows the unit working perfectly and the other shows it to, well, not be.

I realise that we could be discussing how many angels we can get on the head of a pin when all the OP wants is a solution, but do you have any thoughts on the underlying matter?

It is hard to say, but you try checking the health of the WD drive by running DriveDX and install the special USB driver so that DriveDX can communicate properly with the WD drive. There is no guarantee even with the special USB driver that DriveDX can communicate with the WD drive since many external drives block the necessary communication required to access the health information. If DriveDX can access the health information on the WD drive, then post the DriveDX report here using the "Additional Text" icon which looks like a piece of paper.

Unfortunately APFS is a brand new Apple file system so there are no third party utilities to repair it since Apple has not released the necessary APFS documentation. If you cannot access your data then you may need to contact a professional data recovery service such as Drive Savers or Ontrack. Both vendors are recommended by Apple and other OEMs and both provide free estimates.

You should always have backups of all your important data. The more important the data the more backups and copies of it you should have.

The last thing I can think of for diagnostics is to open Terminal and paste: diskutil list

If you copy and paste the output here (redact anything personal) that is something you are likely to be asked for. I wish I had the skill to go further for you. I am at the end of my limited knowledge.

Have you tried rebooting with the drive connected into Safe Mode? If so is the sue replicated there?

On the affected Mac, what does Disk Utility have to say? A screenshot would be useful

Please can we also see a screenshot of Disk Utility without any action taking place, ie as first opened?

Have you been able to try another cable and/or another port on your Mac to run First Aid again?

Pretty much, yes. Thank you. One more piece of data: Top left, the "View" drop down. Please can it show "All Devices"

I know it sounds as if I may have the solution. I am not expert enough. But I'm trying to provide a real expert with enough information to work from.

Oh, this may sound weird, but can you also try another cable and/or another port on your Mac to run First Aid again?

I am inclined towards involving Apple Support, even if it is a paid for service. I think your problem has passed the knowledge level of anyone here. Unless, of course, someone chimes in with an authoritative answer.

Contact Support is the route I would follow