In terms of vendors, it all depends on what tasks need to be accomplished. Obviously, the software should not only block writing to disk, but it also would be helpful to be able to pull the results of the tool into a case management system (like Guidance Software Inc.'s EnCase product line). It's also important that the vendor be able to point to where the tool has been used successfully in legal proceedings, since admissibility is usually a matter of precedent.
A few open source options are starting to appear (search Google for "software write-blockers" to get the latest list), and there are a few utilities like PDBLOCK and RCMP HDL available. NIST is starting to do detailed evaluations of these tools, as well as of hardware write-blockers, which might also be helpful.
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