In his presentation at the Techno Security 2006 conference Robert Botchek summarizes the basics of write blockers. The speaker is president of Tableau, LLC, a manufacturer of write blocking devices.
Write blockers shall prevent accidental tampering of evidence, caused either by operator error or nomal system behavior (think of Windows disk signatures).
Hardware write blockers convert IDE, SATA, SCSI and other interfaces to USB or FireWire. A set of small write blockers and an old laptop could make a great field kit. In addition FireWire and USB support hot-plugging, so you don't have to reboot every time you change the evidence disk.
