UPDATE 1-EMC seeks to prevent former exec from joining HP

*EMC files lawsuit against former unit president Donatelli

*EMC's suit seeks to keep Donatelli from joining rival HP

*Donatelli also files suit to nullify non-compete clause

BOSTON, April 29 (Reuters) - EMC Corp (EMC.N), the biggest maker of corporate data storage equipment, is enmeshed in a legal dispute with the former head of its biggest division who abruptly resigned this week to join rival Hewlett-Packard Co (HPQ.N).

The company and the former president of its storage division, David Donatelli, have filed lawsuits against each other over the terms of a non-compete clause in his employment agreement, EMC spokesman Michael Gallant said on Wednesday.

"Mr. Donatelli's key employee agreement with EMC contains a non-competition clause and others protections for EMC that we intend to enforce," Gallant said.

He said that EMC filed its lawsuit in its home state of Massachusetts after Donatelli filed one in California, where HP is based, seeking to nullify the agreement.

A spokeswoman for HP declined comment. Donatelli could not be reached.

HP named Donatelli as executive vice president of enterprise servers, storage and networking on Tuesday.

He ran a unit whose sales accounted for about three quarters of the $14.9 billion in revenue that EMC rang up last year.

EMC is the top maker of corporate data storage equipment, claiming 22.4 percent of a market worth $20.1 billion last year, according to market researcher IDC. IBM was second with 13.4 percent of the market, followed by HP with a 12.7 percent share. (Reporting by Jim Finkle; Editing Bernard Orr)