This error often occurs after installing Ubuntu 14.* but the solution is easy so follow the steps below to solve the problem.

Serious errors were found while checking the disk drive for /.
press [I] to ignore, press [S] to skip mounting and press [M] to mount manually.

Edit Grub file content in boot manager

Select Ubuntu in boot options and straight away press "e" key which will take you to Grub2 entry. Update line below by changing ro to rw. Pressing Ctrl+X will take you to Ubuntu login screen where you should login to the system.

linux   /boot/vmlinuz-3.13.0-24-generic root=UUID=AAC884AC1F144321 loop=/ubuntu/disks/root.disk ro   quiet splash $vt_handoff

Edit Grub file content in system

Open terminal by pressing Ctrl+Alt+T then type command below. Repeat step "Edit Grub file content in boot manager" as shown above, press Ctrl+X to save and exit.

sudo nano /etc/grub.d/10_lupin

Update Grub to reflect changes

Run first command below in terminal. Once finished, reboot system with the second command. That should be it!

sudo update-grub
sudo reboot