macOS can't repair the disk while first aid shows no errors (Seagate)

I have seen that error message on external drives when they are starting to (physically) fail. Remember that Disk Utility is checking the file system on the drive, which may be ok, but the drive could be starting to physically fail. The physical condition can be checked with DriveDX, which can be used in a free mode for a short time. I ended up purchasing it because I found it so useful. It can examine the physical condition of both internal and external drives, provide you with information on physical errors, lifetime left on a drive, and temperature (running hot is another symptom of a failing drive), plus details on all the SMART parameters.

When I saw that error on an external drive, I have never been able to rescue such a drive or repair it.

Can you still both read and write to the drive?

In any case, I would treat this as an unreliable drive unless you can run the long extended tests in DriveDX (not just the short test) and only if it get good scores on all the physical drive parameters.

By the way, Catalina can work well with both HFS+ and APFS external drives used for storage.

P.S. to HWTech's note about using DriveDX -- in addition to the SMART parameters and other info provided by DriveDX, I suggest that you run both the short and long DriveDX tests on the drive. I have had drives that were fine for SMART parameters and passed the DriveDX short test but failed (consistently) the long test (always failing in the same location).

Also, what is the condition of the drive now? Can you read and write to it normally? Is it behaving normally (speed, noises ...)? Have you tried using a different cable? Sometimes a bad cable (or a good cable not fully plugged in) can cause things like this, although the more common symptom of a bad cable is a message that the drive was ejected improperly.

None of the usual hard drive health attributes are showing any problems. However, the read & write speed reported is extremely slow even for a hard drive (especially the write speed).

Have you tried using the other USB port? Or another USB cable?

Disk Utility has never been very good at repairing file system issues plus I've seen First Aid actually lie to me about the status of a file system scan. Even if First Aid shows everything as "Ok" click "Show Details" and manually scroll back through the report to see if there are any unfixed errors listed (more of a curiosity to see if is just hiding the problem from you). You could try using the paid app Disk Warrior to try to fix the HFS+ file system errors, but there is no guarantee it will do any better although I've found DW to do an extremely good job, but it won't repair APFS volumes. The other option is to do what the original error message mentioned and erase the physical drive as GUID partition and MacOS Extended (Journaled), but of course you will need to temporarily transfer the files from the external Seagate drive to another location.

Please use DriveDx instead and post the report here using the "Additional Text" icon which looks like a piece of paper (you first need to save the report, then open the report in TextEdit and copy & paste the report into the "Additional Text" box). The problem with your screenshot and the other drive health reporting apps is they don't always display the information in an easy to read format and your screenshot is missing lots of important health information.

You must select the "Seagate" item on the left pane and a "Save Report" button should appear on the top right of the app. If it does not, then click the "Health Indicators" item for Seagate. It has been a while since I used DriveDx myself.

anya-ji Said:

"macOS can't repair the disk while first aid shows no errors (Seagate): I'm using a 1TB Seagate external hard drive with Catalina 10.15. It's formatted to Mac OS Extended.[...]"

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Why the Error?:

Catalina uses APFS for File System type. See Here: File System Formats Available in Disk Utility on Mac - Apple Support. So, it is just not going to be changeable, unless it were APFS.

anya-ji Said:

"But I’m pretty sure I was able to copy my files onto the hard drive before, and i didn’t change anything after that... Why is that?"

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APFS File System:

Indeed you can files to it. You just cannot change them. It all has the do with the File System type being used.