This recent Court of Appeals Case, Wyly v Milberg Weiss Bershad & Schulman, LLP
2009 NY Slip Op 03628 ; Decided on May 7, 2009 ; Court of Appeals  discusses the rights of a client to "its" case file for use in a later proceeding…which is often a legal malpractice action.  Wyley is the offshoot  of the Computer Associates class actions, and one in which millions of dollars were at stake.
The rule in New York concerning whether a client is entitled to look at and copy a file  is generally set forth in Matter of Sage Realty Corp. v Proskauer Rose Goetz & Mendelsohn, 91 NY2d 30 [1997]  The Court of Appeals held that the joined the "majority of courts and State legal ethics advisory bodies," which take the position that "upon termination of the attorney-client relationship, where no claim for unpaid legal fees is outstanding," a client is "presumptively accord[ed] . . . full access to the entire attorney’s file on a represented matter with narrow exceptions" (Sage Realty, 91 NY2d at 34).

"The petitioners in Sage Realty retained Proskauer to represent them in a multi-million dollar mortgage financing and a restructuring of ownership interests, all involving New York City properties. After a falling out with Proskauer, the petitioners retained a different firm to represent them in the transaction. Proskauer refused to turn over certain documents relating to its representation of the petitioners, prompting them to commence a special proceeding to recover the documents. "
 

A recurring theme in legal malpractice is how to obtain the documents from the underlying representation.  A confounding factor is that often there is a fee dispute between the plaintiff and its prior counsel, and the prior counsel refuse to allow plaintiff a copy of the file under the authority of a Judiciary Law retaining lien.  Sage remains the authority in this area.