If your SD card is not visible on your iMac but works fine on your MacBook, the problem may relate to iMac's SD card slot, the internal SD card reader, an outdated disk driver, macOS corruption, or other system-wide issues. To fix this problem, you need to ensure a good connection between your SD card and your iMac, update the disk driver, reset NVRAM, etc.
In the first place, to help narrow down the issue, I want to know whether you connect the SD card to your MacBook with the same card reader and if the SD card works fine there. If your answer is yes, then it is confirmed that both the SD card and the card reader function well. It should only be problems related to your iMac.
When an SD card not working on Mac, or the SD card not mounting on Mac, we can analyze the problem from three aspects: the SD card, the connection, and your iMac. As the SD card is already ruled out, then it comes to the connection issue. If you straightforwardly insert the SD card into your iMac's SD card slot, you should make sure the slot is compatible and working. If you connect the SD card reader via a USB port, you can change another USB port on your iMac to see whether it helps.
In case there is no problem with the connection, the SD card is still not visible or mounted on your iMac, then you can try other working solutions:
- Update the disk driver. Perhaps the disk driver on your iMac is outdated or corrupted, then it leads to the SDB card invisible and not mounting issue. You can update the disk drive by updating macOS from the Apple menu > System > Preferences > Software Update.
- Restart your iMac. It is probably the system errors that make the SD card malfunction on your iMac. Then restarting it may fix the problem with ease.
- Reset NVRAM. NVRAM stores setting information from your Mac model and its connected devices. When your SD card is invisible and not mounting, you can try to reset NVRAM on Intel Mac by holding the Option + Command + P + R keys while restarting the Mac. On an Apple Silicon Mac, you just need to restart the Mac to reset NVRAM easily.