In a strongly worded letter dated April 9, Western Digital, which operates a semiconductor plant with Toshiba in Japan, said the transfer of the venture's rights to a new chip unit which was split off recently without the U.S. firm's consent was a "very serious breach".
The letter, seen by Reuters, also said the U.S. firm would not sit idly by while Toshiba "runs roughshod" over its rights, although it did not threaten legal action.
The move, seen as a play by the California-based company to strengthen its hand in talks, is set to complicate the auction of the prized unit - a sale essential to Toshiba's plans to cover writedowns at U.S. nuclear unit Westinghouse that have plunged it into crisis.
"The only thing holding Toshiba up at the moment is the sale of the memory chip business. This definitely increases the uncertainty around everything that is happening," said an executive at one of Toshiba's main banks.