Fix boot hangs with /dev/sda1: clean ... files ... blocks in Ubuntu

dev-sda-clean

If you have switched graphic card driver from Nvidia to Nouveau in Ubuntu, you might get this message after boot, where it just hangs:

/dev/sda1: clean, ***/**** files, *****/**** blocks

Solution:
Click ESC during boot
Select Ubuntu with Linux ... (recovery mode)
Select Resume normal boot
You should be able to log in, switch back to Nvidia end reboot