Samsung have just released a firmware fix to address the EVO 840 SSD data access issue. The drives were plagued with a problem causing older data to be read incredibly slowly, much to the annoyance of many users.
We recommend that any users wishing to perform the firmware fix back up their data (twice) beforehand. If drives are configured in a RAID formation they will need to separated before performing the update.
Samsung state that the update can take up to an hour depending on the amount of data on the drive.
Grab the firmware here.