The HD Tach sequential read test is a little bit different from other benchmarks.
Most benchmarks create a file on the hard drive and test within that file.
The problem is that modern hard drives use a zone bit recording technique that allows different read speeds depending on where the data is located. Data on the outside of the drive is much faster than data recorded on the inside. HD Tach reads from areas all over the hard drive and reports an average speed. It also logs the read speeds to a text file that you can load into a spreadsheet and graph to visually read the results of the test. In addition to sequential read, HD Tach tests the drive's random access time.