The American Civil Liberties Union is also on the record urging the appointment of a special master. They point out (quite rightly I think) that given the dense nature of digital evidence in the information age, that prosecutors tend to sweep up much more in searches than they used to when files were on paper.
Prosecutors are essentially in possession of the entire documented history of Cohen’s law practice, far beyond the scope of the search warrant.
This is unavoidable of course, given the nature of digital files.
The ACLU points out that all of us ought to be able to expect a fair referee to eliminate material not under investigation, should any of us ever be subject to digital file search.