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. Random access is the true measure of seek speed. Many drives advertise sub 10 millisecond seek speeds, but seek speeds are misleading.
Finally, HD Tach tests the drive burst speed. The burst speed is the speed that data can be accessed from the drive's on-board read-ahead memory. This measures the speed of the drive and controller interface. The registered version of HD Tach adds write testing and Windows NT/2000 support.