Q:

GRUB available, but can't boot (end_request: I/O error)

I run Ubuntu 14.04 on Asus S400 laptop that has /root at the 20Gb SSD drive.  While /home directory is mounted onto the SSD drive, the directories are placed onto a 500Gb drive via symlinks. 
When I boot the computer, it goes into GRUB and offers to load Ubuntu or run memory tests (memtest+86).  I go to Advanced Options for Ubuntu and choose to run the recovery mode.  When I go to fsck (Check all file systems) I'm getting the following errors:
...
ata2.00: status: { DRDY ERR }
ata2.0: error: { UNC }
end_request: I/O error, dev sdb, sector 41946208

However, when I boot from a Ubuntu USB key I can see the drive directory structure as well as run tests on it. 
Any suggestions on how to resolve the problem?  Any picks on diagnostics and recovery tools are very appreciated. 

A:

Turns out it actually was the SSD mounted to / giving problems.  I've figured it out using the Live CD Recovery guide with
$ sudo fdisk -l /dev/sdb

Then I had to reinstall Ubuntu back onto the SSD using a USB key (see this). 
After the reboot, the GRUB turned out to be broken.  I've fixed that using the boot-repair tool that I installed onto the live Ubuntu USB key from a PPA (see this).