Photos suddenly fails to import images to external SSD library with unknown error after working minutes before!

In addition to trying a new Library as Yer_Man suggests, try these steps:

• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to temporarily move your Library out of your own user's Pictures folder up to the general "Users" folder. 

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

It really looks like the Library itself has some corruption. I can't tell from this discussion if you are using iCloud Photos. If you are, do you see all your pictures at iCloud.com? If so, then your best bet is to create a new Library; make it your System Library, and wait for it to fill from iCloud Photos. If you don't use iCloud Photos, it is probably worth the three bucks for a month's rental to do this. iCloud Photos avoids corruptions, so you end up with a clean copy. As Yer_Man says, this will take over a week, probably, with 600 GB. See this:

Make a New System Library connected to iCloud - Apple Community

This may require a new external drive, since the Library is so large. 

By the way, have you check the Permissions on the hard drive? This is not usually a problem, but it sometimes comes up.  Click on the drive icon, and open the Info window with command-i. Go to the bottom:

If you are adverse to using iCloud, you could create a new Photos Library and use File>Import to bring in all the pictures into it. I think that the Import process avoids corruption, but you lose some information, like faces-- I think.

You could also try using the trusted 3rd party app PowerPhotos ($40.) This makes transferring pictures from one Library to another pretty easy. You could start with the free version.

Another option is to Reconstruct the Corrupt Photos Library.

Reconstructing a Corrupt Photos Library - Apple Community

This is sort of a nuclear option. Do this only on a Backup!

Let us know…

Does it happen with all images and, for example, with images straight from iPhone or with plain vanilla .jpg?

Do you use any 3rd party disk utilities like Samsung Magician etc? Usually those are not needed and Disk Utility is all you need.

Do you use any 3rd party virus apps, "cleaners" or VPN because they produce obscure errors like this? macOS built-in security is all you need.

An EtreCheck report would eliminate much guesswork, so paste the complete report here using "Additional text" option, so we can examine it for possible clues:

Using EtreCheck to Troubleshoot Potential… - Apple Community

As a test:

Create a new library on the same disk. Can you import a few images into that one?

Yes! Created a new one called TestLibrary in the same folder as the systems library, and it imported a couple of JPEGs without issue.

In addition to trying a new Library as Yer_Man• suggests, try these steps:

Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support•  

Booted in Safe Mode. (verified in System Information).

When I first opened Photos, it went into "restoring library" for a good 5 minutes or more before starting up.

result: :-( same error

The only progress so far is creating a new library in the exact same external folder does work - I am able to import images into that new library.

So... is there anything that can be done with the existing systems library to get it to import photos??

This tells us that the issue is with the actual Library and nothing to do with the disk or anything else, I tink You have tried to rebuild the library with the app's own tools?

Might be time to create a new empty library and sync that with iCloud which will take a very long time...

All images types that I have : TIFF, JPG, and PNG, including HEIC "straight from iPhone"

No. Tried Disk Utility "First Aid" on the external SSD where the library is, and it found no issues.

No. no VPN or virus apps on this machine

Will try the EtreCheck, but never heard of it. Assuming its legit?

BTW, just noticed that I am now NOT ABLE to move an image from one album to another. ??????

Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to temporarily move your Library out of your own user's Pictures folder up to the general "Users" folder. 

Let us know what happens…

so my iMac has separate accounts for my wife and kids on it as well. From my wife's Photos, using the same system library on the external SSD, I got the same error.

Thanks for the report but nothing obvious catches my eye. I let others to chime in.

further to the above comment, now I am able to move images from one album to another. No idea what happened to cause this change.