Yosemite freeze at boot - no safe boot + I/O error
I/O problems are a good indicator that your drive is past its best. Replace the drive or do the following...

Back up everything of value via Target Disk Mode (if possible) then boot your MacBook from a USB or DVD installer. Run Disk Utility, selecting the secure erase/zero fill option and wipe your drive.

This will issue an ATA secure erase command that will fill every allocation block with a zero, automatically mapping out any bad blocks/sectors that will be causing your I/O problem. This is the modern equivalent of the old low-level format, and while there is some debate on the subject I use this method regularly and it works well - assuming the drive isn't riddled with bad blocks.

All FSCK does is check the file structure, it doesn't scan the drive surface to determine if the drive has any damaged sectors. Even the drive's internal SMART diagnostics may indicate all is well when in actual fact the drive is on its way out.