TCD Labs' products have been acquired by Simpli Software.

HD Tach is a physical performance hard drive test for Windows 95/98/ME and Windows NT/2000.  In Windows 9X/ME it uses a special kernel mode VXD to get maximum accuracy by bypassing the file system.  A similar mechanism is used in Windows NT/2000.

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.  Access time is the time it really takes to read data, not just the time it takes to move the head to the proper cylinder.  To calculate access time the software must read a single sector off of the hard drive.  By reading a sector the drive can not respond to the command until the sector is available, so rotational latency + seek time = access time.

A 7200 rpm drive has a rotational latency of 4.15 ms on average.  A 4500 rpm drive has a rotational latency of 6.67 ms on average.  So a 7200 rpm drive with a seek of 12.5ms has an access time that is just as fast as a 4500 rpm drive with a seek of 10.0 ms.

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.  Currently UltraDMA/66 IDE drives can reach 66mb/s. Ultra2LVD SCSI drives burst up to 80mb/s.