Whenever the SD card is connected to the Mac system via card reader or SD slot, it will show up in the Finder, Disk Utility, or desktop. When Mac users do not see the SD card shows up in any of the given locations, it means it is not set up correctly. Besides, there can be various reasons for the SD card not showing up on Mac devices such as Viruses, Malware, corrupted SD card, and damaged card reading slot.

Quick Check on SD Card
Before proceeding, there are a few small checks which also can be performed by Mac users which are fast and effective to the SD card not showing up on Mac issue:

Wipe the SD card with a clean and dry cloth to remove any dirt, dust, or grease.
Try another SD card reader or slot to check if its working or not.
Suppose that you are using a microSD, make sure to unlock it safely.
Make sure the contact points of the SD card are correctly position and facing down before inserting into the SD card slot on the Mac device. SD slot can be thin and wide, so Mac users must be careful when connecting
the SD card.
Most importantly, there can be a high risk of data loss from SD card when you are fixing the issue. Therefore, to prevent permanent loss of data, we suggest you go through together the steps below to recover all essential data from the SD card by using the reliable and free Mac data recovery tool.

Solution 3: Open Disk Utility to Fix ‘SD Card Not Showing Up Mac’ Issue
If the SD card is still not show-up after using the solutions above, then use Disk Utility to resolve the problem. Disk Utility can be launched by going to the Launchpad, followed by Others, after doing this, you will come across one of the situations below:

SD card not showing up in Disk Utility Sidebar.
SD card unmounted in Disk Utility.
Situation 1: SD Card Not Showing Up In Disk Utility Sidebar
A sidebar can be seen by default with all volumes and disks at the left side of the Disk Utility window but when the settings are changed, you will not be able to see the sidebar at the left side. This might make you think that your SD card is not appearing.

Situation 2: SD Card Unmounted or Grayed Out in Disk Utility by Using First Aid
Refer here for the best solution to fix SD Card not showing up on Mac with the best data recovery software for Mac.