My SD card reader refused to mount after macOS Sequoia update

This may sound stupid but I noticed after plugging it in, something appears on the screen then quickly disappears. I believe it's a prompt to allow it to be read/mounted that is quickly disappearing. Maybe like the Allow/Trust pop up that shows up when you connect an iPhone/monitor to the Mac. It disappears so quickly that you can't see it.

So I plugged it in again and kept hitting Enter and boom. I can now see my Canon SD Card. This is clearly a Sequoia bug. Cause it's happened on two different Macs and I have been able to do the same thing using a different SD Card.

Actually after digging further into Sequoia, here's a suggestion:

If you go to Settings & Search 'Gatekeeper'. Set 'Allow Accessories to Connect' > "Always" and see if you can mount your SD Card.

I wouldn't keep this off though...

Many have reported that the built in card readers on iMacs have issues. Mine(2019) would not read unless I rebooted the iMac with the SD card already inserted. However, an external card reader worked fine. My iMac started to work after I did the next bOS update

SDHC card (16gb) which was working perfectly before Sequoia 15.0.1 stopped showing in sidebar. Could see it in Disk Utility, but mount was greyed out. I tried a new SDXC card (64gb) and it worked perfectly. SDXC reader isn't reading SDHC cards, it seems

SanDisk SDHC formatted in Fujifilm camera, worked perfectly before Sequoia update. SDXC also formatted in Fuji camera, works perfectly now.

This didn't work for me either.

The only workaround I found that allowed the SD card to mount is ReStart with the SD card in the slot.

All previous OS versions were seamless; this latest OS and Apple's non-response to the issue, which has been raised since October, is, quite frankly, pathetic.

Yes! SDHC now shows up in side bar and Image Edit now! Great. I had just found a way around the previous problem: I plugged the camera into MacBook, camera showed up in side bar and in image edit. As the card was in the camera, I was able to access and download the photos. (Fujifilm xt5; but this method didn’t work with a Sony!)

My wife has the issue and she is using a M1 IMac. She could no longer load the SD card from the camera, using an external card reader all is OK. The error she got every time was that the Media is not readable etc..The strange thing is i have a 2019 Intel IMac and for me things always worked OK so the Sequoia udates did not make a difference. She is using an external card reader for now as 15.1 still did not fix it for her.

All my SanDisk SDHC cards read fine (Fat32 format) with 15.0.1. on my Mac Studio. What card are you using and how is it formatted?

I too have tried to mount an SD card in the slot, then through two different card readers but it just won't mount - in one reader I could see it, but it would not mount. I tried an older computer and it read the card no problem. Until I upgraded to Sequoia I was putting cards in all the time. What is the fix for this?

SD card formatted to FAT32, and iMac updated to OS 15.1, but the SD card is still not mounting... can see it in Disk Utilities, but greyed out (as mentioned above). If I try via the camera, is it also greyed out... can't find the 'Allow Accessories to Connect' in settings either, despite searching. Any other options please? I can't reformat the card, as I have photos I need on it! Thanks.

Thank you both (uwe294 & tbirdvet) for your help! I've now managed to get it to work.... been having problems with Photos too, with the various error codes, it wanting to do a repair etc... but seems to also be working currently! I think my iMac (2019, 21.5") is probably just a bit old now!

It is frustrating!! It seems to be completely erratic with my Mac, as sometimes it works fine, then others, it doesn't mount, meaning a reboot, which wastes so much time. I do hope Apple will provide a proper fix soon!!