The most effective way yet to fix the Broadcom card reader not showing up on MacBook running Kubuntu is using command lines. Follow the detailed prompts below.
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.