Hi, my friend, I have to say, sometimes, the SD card can't be recognized and mounted by macOS properly if you insert the card too quickly or too slowly. So, insert the SD card into the slot on your Mac at a normal speed.
And since you say that the SD card can work well on a PC but only 25% of the time is workable on your iMac, I suspect that the reasons may be the damaged SD card slot on your iMac, SD card corruption, or system bug.
To fix your SD card that not mounting on your Mac, you can try these tricks:
- Try connecting your SD card to your iMac through a compatible SD card reader. The SD card slot on your iMac may have some hardware issues, causing the card to be mounted occasionally.
- Reformat your SD card on your Mac. Remember get off the files on it first.
- Update your Mac to the latest minor update rather than upgrading it to the latest version directly.
If these ways help nothing, go to a local repair center to check if your SD card has hardware issues.