What is happening? I/O error, dev sda, sector 545868115

You need to check and see if your disks are indeed failing. There are command line tools for monitoring SMART data (which is data that the hd reports about it's own health).

sudo apt-get install smartmontools gsmartcontrol

Gsmartcontrol is a gui version of the same tool, and is very easy to use. Select the disk and run a short or long test in addition to viewing any errors the hd is reporting.

To run via the the command line, do the following

sudo smartctl -i /dev/sda 

Make sure SMART is turned on. If not,

sudo smartctl -s on /dev/sda 

Finally, run

sudo smartctl -a /dev/sda 

or if it's a serial ata drive.

sudo smartctl -a -d ata /dev/sda

See this link for more info about smartmontools. http://sourceforge.net/apps/trac/smartmontools/wiki/TocDoc