According to report, Toshiba is suing Western Digital for 120bn yen ($1bn; £835m), accusing it of interfering in the attempted sale of its flash memory business.
But the Japanese giant is offloading the unit, which makes chips for smartphones and laptops, to cover huge losses in its US nuclear division.
It has so far failed to clinch a sale to its preferred buyer, a group of US, South Korean and Japanese investors.
Western Digital, a partner of Toshiba, objects to that proposed deal.
The US firm, which jointly runs Toshiba's main semiconductor plant, has been feuding bitterly with its Japanese partner, and has sought a US court injunction to prevent any deal that does not have its consent.
Toshiba is selling its memory chip unit to cover losses from its nuclear division.