If uploading software from the Arduino IDE or the commandline via avrdude to a microcontroller like an Arduino or an ESP8266 fails due to missing access rights ("Permission denied"), what helps is either
sudo chmod a+rw /dev/ttyUSB0
or adding the user to the group "dialout":
sudo usermod -a -G dialout theuser
Sometimes after a hard shutdown the file system of my laptop is corrupted and Debian starts in maintenance mode.
What helps is repairing the file system using the command
fsck -y /dev/sda1