SD card reader not detected in Ubuntu 16.04

Probably an installation-issue which can be resolved with re-installation?! This would concern package udisks2.

1. Reinstall package udisks2

   sudo apt-get install --reinstall udisks2
   

2. Reboot the machine

   systemctl reboot
   

SD cards should be recognized then, after inserting them (in and out and in cautiously).

If you have installed a Card reader lsusb should give you something like:

Bus 001 Device 005: ID 0bda:0129 Realtek Semiconductor Corp. RTS5129 Card Reader Controller

I am guessing you have Realtek card reader?

Download the Linux driver for your Realtek device for example rts5159 Open a new terminal session

Install the driver. Run the following commands:

cd where-your-driver-was-downloaded-extracted 
make
sudo make install
sudo depmod