Reads entire device and times each read. If the hard drive is going bad, it may take significantly longer to read certain sectors, even though it succeeds and SMART reports no problems. This may indicated impending failure. Additionally, reading the entire disk can trigger the drive's automatic sector replacement for weak sectors, refreshing the data. You may want to tee the output to a file for later review, there may be a lot fo lines for large disks. Run on an idle system, other activity will skew the drive read times.