Toshiba shares plunged in Tokyo on reports it will need to take a larger write down on its U.S. nuclear business and seek cash from its American partner Western Digital Toshiba shares plunged again in Tokyo Thursday following media reports the electronics giant will need to take a larger writedown on its U.S. nuclear business and potentially seek cash from its American partner Western Digital Corp. Japanese media said cost overruns at its CB&I Stone & Webster unit will push writedowns in the newly acquired business to between Y500 billion and Y700 billion , much higher than the company’s original estimate of $87 million.