Write blockers are devices that allow acquisition of information on a drive without creating the possibility of accidentally damaging the drive contents. They do this by allowing read commands to pass but by blocking write commands, hence their name.
There are two ways to build a write-blocker: the blocker can allow all commands to pass from the computer to the drive except for those that are on a particular list. Alternatively, the blocker can specifically block the write commands and let everything else through.
Write blockers may also include drive protection which will limit the speed of a drive attached to the blocker. Drives that run at higher speed work harder (the head moves back and forth more often due to read errors). This added protection could allow drives that can not be read at high speed (UDMA modes) to be read at the slower modes (PIO).
There are two types of write blockers, Native and Tailgate. A Native device uses the same interface on for both in and out, for example a IDE to IDE write block. A Tailgate device uses one interface for one side and a different one for the other, for example a Firewire to SATA write block.
Steve Bress and Mark Menz invented hard drive write blocking (US Patent 6,813,682).
There are both hardware and software write blockers. Some software write blockers are designed for a specific operating system. One designed for Windows will not work on Linux. Most hardware write blockers are software independent.
Hardware Write Blockers
Hardware write blockers can be either IDE-to-IDE or Firewire/USB-to-IDE. Simson prefers the IDE-to-IDE because they deal better with errors on the drive and make it easier to access special information that is only accessible over the IDE interface. You may feel differently.
NIST test results are here: https://www.nist.gov/itl/ssd/software-quality-group/computer-forensics-tool-testing-program-cftt/cftt-technical/hardware
MyKey Technology, Inc. NoWrite FPU and FlashBlock II 1.8"/2.5"/3.5"/ IDE to IDE, FireWire/USB to IDE & SATA, all media types - NIST Ver. 2 accepted
Tableau write blockers for IDE, SATA, SCSI, USB NIST Ver. 1 accepted https://www.guidancesoftware.com/products/Pages/tableau/overview.aspx
WiebeTech write-blockers for almost any disk drive: 2.5"/3.5" IDE, SCSI, SATA, ... https://wiebetech.com/products/ NIST Ver. 1 accepted
EPOS WriteProtector https://www.epos.ua/view.php/en/products_epos_write_protector
Software Write Blockers
Software write blockers can be either tailored to an individual operating system or can be an independent boot disk. Their main upsides are with ease of use, since they are on a CD and do not require you to open up the case, and speed since they do not become a bottle neck.
SAFE Block XP SAFE Block XP is a software-based write blocker designed for the Windows XP Operating System. It comes in both 32 and 64 bit options. NIST Ver. 1.2 accepted