Multiple reasons will make your SD card not show up on your Mac mini. For example, your SD card has physical damage or logical corruption, the system files of your SD card are corrupted, you incorrectly ejected your SD card last time, your SD card has been locked, etc.
You can try to mount your SD card using third-party software like iBoysoft DiskGeeker, which can help you check for errors on inaccessible, unreadable, or corrupted drives connected to your Mac so that you can fix them.
- Free download and launch iBoysoft DiskGeeker.
- Connect your SD card to your Mac mini.
- Wait for it to mount and check whether you can see it on the iBoysoft DiskGeeker left sidebar.
In addition, you can try to fix your SD card by running First Aid in Disk Utility. If it's not working, try to recover your data from the SD card and format your SD card after recovering. iBoysoft Data Recovery for Mac can help you recover data from the SD card securely and effortlessly so that you can have a try.