(Corrects in paragraph 6 that unit Toshiba agreed to sell is flash memory, not Westinghouse)

Sept 20 (Reuters) - Western Digital Corp filed its latest legal action against joint-venture chip partner Toshiba Corp for moving to invest in a new flash memory production line without its help, the U.S. company said on Wednesday.

The new arbitration requests, filed in the International Court of Arbitration that oversees the companies' agreement, seeks to stop Toshiba from investing in the so-called Fab 6 facility in Yokkaichi, Japan, unless Western Digital's subsidiary Sandisk was also allowed to invest.

In a statement, Western Digital asserted it had the right to co-invest and had said it was "disappointed" in Toshiba's decision in August to proceed on its own.

The court, a branch of the International Chamber of Commerce, is an institution for the resolution of international commercial disputes.

The companies have battled for months over Toshiba's efforts to sell its memory unit to raise cash to plug a hole in its finances caused by its bankrupt U.S. nuclear unit Westinghouse.

Toshiba officials said earlier on Wednesday they had sealed a deal to sell the flash memory unit for $18 billion to a consortium led by U.S. private equity firm Bain Capital LP that also included Apple Inc, helping to keep the Japanese conglomerate's listing on the Tokyo stock exchange.

Toshiba representatives did not immediately return a request for comment. (Reporting by Stephen Nellis in San Francisco; Editing by Richard Chang)