Measures both sustained and burst data transfer rates of your hard disks, cd/dvd-roms and floppy. Realtime graphical display.

What is the chain through which the data flows to reach hd_speed?
Simplified: drive platters -> driver controller (including it's own cache) -> ide/scsi/sata connection -> host controller (for ide&sata, usually part of the motherboard chipset) -> drivers for hdd controller -> hd_speed

What method does HD_Speed use to read/write ?
The following Win32 API calls: CreateFile(devicename,...), ReadFile() and WriteFile(). See Microsoft's PlatformSDK for more info.

What relevance is the offset setting ?
The transfer speed of most disk types varies at different points. eg, harddrives are usually fastest at the start, and slower further on. CD-Roms are typically the opposite