SD card reader not showing up on MacBook running Kubuntu, how to fix?

Step 1. On your MacBook running Linux, open the terminal by clicking on the Activities from the top-left corner and type in terminal/shell/prompt. Click on the terminal icon and open it for writing command lines.

Step 2. Type in the following prompts:

$ sudo touch /etc/rc.local  
$ sudo chmod +x /etc/rc.local  
$ sudo nano /etc/rc.local  
### add this line:  
setpci -s 00:1c.2 0x50.B=0x41  
[ctrl-o to write out, enter to save, ctrl-x to exit]

$ sudo nano /etc/apm/resume.d/21aspm  
### add the same line  
setpci -s 00:1c.2 0x50.B=0x41  
[ctrl-o, enter, ctrl-x]

$ sudo nano /etc/modprobe.d/sdhci.conf:  
### add the line:  
options sdhci debug_quirks2=4  
[ctrl-o, enter, ctrl-x]

### regenerate initrd  
$ sudo update-initramfs -u -k all

### reload shdhci module  
$ sudo rmmod sdhci-pci sdhci  
$ sudo modprobe sdhci  
$ sudo modprobe sdhci-pci

### reboot

Step 3. Your Mac restarts now. If unfortunately, your SD card reader is not working still, you can try the following command lines.

$ sudo setpci -s 00:1c.2 0x50.B=0x41

Other methods such as hardware check may work, but running prompts is considered the most working way to fix this issue.