howaa wrote: can you tell me where to find that system log?
He already told you where it was: /var/log/system.log
Just copy and paste the following command in a Terminal (/Applications/Utilities/Terminal) and then press Enter.
grep "I/O error" /var/log/system.log
It will output to the Terminal all lines in the system.log file that has "I/O error" in it. Output such as kernel[0]: disk0s4: I/O error is indicative of a falling HDD. If you have output such as that and aren't backed up then do it before doing anything else! 