I thoughtlessly removed my SD card from the slot of the card reader while the SD card was in use in gparted and the disk-utility. After I put it in again, it was not visible anymore. It is the same situation with another SD card. I reopened the disk-utility and the complete card reader doesn't appear anymore. I tried to restart the computer but the situation remains the same - The card reader doesn't appear in the disk-utility. How can I reactivate it?
After the problem could not be solved by a simple reboot, the following procedure was successful: Switching off the computer, then removing any power source for 5 min (simply plugging off power supply cord). After reconnecting and booting, the card reader was detected again.