TOKYO, Jun 7 (Reuters) Toshiba Corp on Wednesday asked Western Digital Corp once again to stop challenging the Japanese conglomerate's plans to sell its chip business. "Toshiba encourages Western Digital to redirect the considerable efforts that it has put into disrupting Toshiba's sale process into more productive channels that benefit both Toshiba and SanDisk," Toshiba lawyers said in a letter dated June 7. SanDisk, which was acquired by Western Digital last year, owns a 49.9 percent stake in a joint venture with Toshiba. REUTERS PY 1502