Photos not exporting pictures, Unknown error (4)

There is a group of recent problems with Photos that, in my opinion, originated from the same source problem. I was working with a local library not connected to iCloud. I have encountered "'Error downloading slideshow photos" followed by "Internet Connection required" when tried creating a slideshow from an album. I did elimination search, adding each time a small group of photos, until I found the photo that causes the problem. I tried to edit the photo and encountered an error "cannot Start Editing. Photos cannot load adjustments for this image". Tried to export the photo and got "Unknown Error(4)". Stayed in editing mode, using the arrow to move quickly to the next photo and found several others with the sam error.

Searched here and found a thread https://discussions.apple.com/thread/250771558 without solution.

So, the bottom line:

There is a bug in Catalina which makes some of the modifications done on photos in the past unrecognized by photos. Since a display version or exported version of a photo is created by taking the master and apply the editing modification on it, such photos cause all sorts of problems. I will report this but to Apple.

Interesting. I get the same 'Unknown error (4)' when trying to export some pictures. However, I am not even able to 'Edit with' and then 'Preview' because that results in 'An error occurred while preparing external edit." See also https://discussions.apple.com/thread/250771558, which may be related.

I'm pretty sure it's an issue related with new partition disk on Catalina (now Mac HD is partitioned in two) because this message in Photos preferences: application doesn't recognize my photo's library as system library and iCloud functions are available only fro system photo library (I'm using Spanish MacOs sorry).

Have you tried to make the library the System Photo Library in the Fotos > Preferencias > General?

Click: " Usar como fototeca del sistema".

This field should be dimmed, to show that the library is the System Photos Library.

If it does not stay grey, your system library may be on a volume, that is not compatible with Photos.

I have just tried to export a number of photos from an album. 5 of them have failed to export whereas many others have not. I can see no correlation between those that worked and those that failed.

In case it was the Max Pixels setting, I tried increasing that value of 1024 to 80000 setting in all 3 options to no avail. Any ideas anyone?

The "unknown error" is very typical for a library that has been damaged by cleaning applications. If you are seeing the problem only with older photos it might be caused by running apps like CleanMy Mac on the library several years ago.

But it might also be a problem of photos with external edits, where the external editor returned the edited version in a format, that is no longer supported in Catalina.

Or simply photos in an unsupported format, that have been imported or edited while using Aperture or iPhoto, and are now incompatible with Photos 5. I found some PSD files in my library, imported with Aperture, that Photos can no longer open.

In that case try, if you can open the image in Photos, if you revert it to the Original.

Nothing of the above applies to my case. It is totally random which photos fail and which do not. I have been trying to make a RESNAP photo album of a recent holiday in the French Alps. I had previously imported all the photos from my camera, none had undergone any of the issues in the reply, and the wanted to select ones to export to upload into the RESNAP album. Approximately 50% of the photos failed UNKNOWN ERROR (4)! My only recourse was to screen shot every failing image to create my RESNAP album. Moreover, I have tried this on many other selections of photos and this approximate 50% failure rate is what I am getting wherever I select photos from.

As a computer programmer once, we would have been sacked if we EVER output a message like this without giving the EXPLANATION of the error code, 4 in this case. Nor would there be a variety of reasons why this code was output, which is the BIGGEST SIN in this case. We would always the exact LOCATION in the code where it was detected saying what the PARAMETER was that was being tested and the VALUE was that had been found that was not unexpected.

This is not what I have come to expect from APPLE! Let’s hope Apple can put some better diagnostics in to help pin down and not have MULTIPLE REASONS for outputting a single ERROR CODE VALUE. There are enough different numbers in the universe to avoid this ever happening. 😀

One last plea...please put the name of the file that is reporting the UNKNOWN ERROR (4). When you are trying to export 300 files and 150 fail finding out which they are is an absolute nightmare. PLEASE ADD THE FILE NAME to the error report

Same here, I can't export any file from Apple Photos on Mac Book Pro.

Catalina Photos. The gift that keeps on giving

It's a bug, alright. And I'm surprised Apple didn't notice it. Just shows that they don't care as much as they used to.

Hi Léonie, it works! I could export photos using my library as System Photo Library again: it seems Catalina loose this option!

Thank you for your suggestion!!!

Reverting to the original results in an eternally spinning beach ball.

AND its date and time . Thanks