iMac will not read sd card

Welcome to the Apple Support Communities.

Please see: About the SD and SDXC card slot on your Mac - Apple Support.

Cheers,

Jack

Assuming that it's not the SD card itself it may be the actual SD card reader. Someone I know recently had to purchase an external USB SD card reader for their Macbook due to the built-in one not working anymore.

I have the same issue. I had no problems with SD cards at first. After some months, I was having to jiggle any SD card I used and/or reseat it and/or reboot. Those worked for a few months. It has reached the point where none of those fixes work. It will no longer read any SD card, yet I can read them on other computers. I know it is not the external reader, because I replaced it and have problems with every external reader I try at this point. It is not the SD card, not the reader, which leaves the MacBook Pro as the culprit. I need an answer as to how to fix the problem. Thank you.

I have the same problem with several SD cards. I just wiggle it around some or reseat it a few times and eventually my Mac reads it. Pathetic "fix" but it usually works.

If the SD card is from a camera and you have the correct cable which came with the camera you could try plugging directly into a USB port on the iMac and downloading from the camera.