Issue with importing photos from an SD card reader onto my late 2024 Apple iMac.

JonathanCambs wrote:

The external drive is correctly formatted. The SD card reader in question has been working without any problem for over two years, as has the Transcend portable drive connected to one of the USB 3.0 ports on the iMac. The latter is also correctly formatted. The issue has only started happening in the last week or so.

"has been working without any problem for over two years," just isn't very valuable diagnostic information-- almost everything, just before it breaks, was working without problems right before that.

It could be the cable; it could be the reader; it could be the card itself. I can say that my MacBook on Tahoe 26.3 reads my SD card from my Nikon just as it always has.

Do you see the SD card appearing in the Finder Sidebar? in the Photos sidebar? in the Disk Utility ap?

To diagnose possible glitches and narrow things down, try these steps:

• Restart Photos
• Restart the computer (of course)
• 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 

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• 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…

What format is the external drive? It needs to be formatted Mac OS X Extended, Journaled or apfs.

JonathanCambs wrote: … However, if I then shut down and restart the computer, and re-open the Photos app, the same photos then import fine. Does that give any clues as to the root of the issue? I am starting by buying a new SD card reader, to see if this has any impact.

That does make me wonder about the results of using the Safe Mode or a different user account. It would be worthwhile to try the things I suggested.

Have you used any "cleaner" apps or "virus catcher" apps? A VPN? Those things are largely worthless, and they are known to interfere with Mac operations.

Is it a full size card or micro with adapter?

Can you use the finder to look at the DCIM folder?

You might have to run Image Capture directly. I am not sure why, but sometimes Image Capture (Which does the importing behinds the scenes) forgets that it should pass the images off to Photos.

Most are bad, though some are just useless. Cleaners and anti-virus programs have been known to corrupt the rather delicate Photos Libraries.

Some VPNs are required at work, and they are usually OK. More extensive VPS just mess things up.

It might be worthwhile to use a different user account that you know hasn't been corrupted.

That's great!