Shares of Western Digital Corp. surged Friday on word that the Irvine-based disk drive maker has resumed some of its operations in Thailand and raised its revenue outlook for the current quarter.

Investors sent shares up more than 9% in midday New York trading, to a market value of nearly $7.5 billion, on the better-than-expected outlook.

All of Western Digital’s production plants in Thailand had been closed for months in the wake of the worst flooding disaster to hit the region in decades.

In the December quarter, the company upped its revenue projection to at least $1.8 billion, with gross margins between 18% to 23%.

Western Digital last month had lowered its projection on revenue between $1 billion and $1.25 billion, down from $2.47 billion a year earlier.

“The passion, perseverance, ingenuity and execution exhibited by the WD team has been extraordinary and enabled us to make substantial progress in partially restoring our operations in Thailand well in advance of our earliest expectations when the floods hit,” said John Coyne, president and chief executive of Western Digital.

The developments prompted at least three brokerages to up its price target today on Western Digital stock as some analysts said the company will recover lost market share quicker than expected.

Western Digital is in jeopardy of losing its title as the world’s largest disk drive maker this quarter to Cupertino-based rival Seagate Technologies LLC.

The company has held the market share lead over Seagate for nearly two years and could slip to No. 3 or No. 4 worldwide, according to El Segundo-based market tracker iSuppli Corp., a unit of IHS Inc.

In the announcement Thursday night, the company outlined progress in a number of areas, including:

• Restarting production of hard drives this week in one of its buildings in Bang Pa-in, Thailand, a week ahead of schedule. The plant had been submerged in six feet of water since October 15. It was pumped dry on Nov. 17, main power was restored Nov. 26 and production restarted Nov. 30.

• Removing all submerged slider manufacturing equipment from the Bang Pa-in building for assessment, decontamination and refurbishment, as well as beginning decontamination and restoration of its remaining buildings at the complex.

Western Digital expects to restart head slider production in the March quarter in Thailand and at a new slider fabrication plant in Malaysia.

Its other Thailand hard drive plant at Navanakorn remains under two feet of water. The complex is expected to be pumped dry in the next few weeks, before decontamination and refurbishment starts, according to executives.

The company’s flooded buildings in the region house assembly and testing operations along with a substantial majority of the company’s slider fabrication, which accounts for a key component in disk drives.

Western Digital ships about 60% of its disk drives from Thailand, where it employs some 37,000 people.

Its disk drives go into computers, external storage devices, corporate networks and consumer electronics.

Operating expenses are projecting to be about $265 million, excluding charges related to the floods, acquisition-related expenses and litigation.

The cost to restore operations and related expenses are expected to be in the range of $225 million to $275 million for the December quarter, not including insurance recovery.

The company expects to file a property damage claim of at least $50 million and an additional claim for business interruption during the December quarter.

It received some relief earlier this week when the Thai government announced it would waive import fees on some products to help companies operating in the country affected by widespread flooding and related damage.

In connection with its planned acquisition of San Jose-based Hitachi Global Storage Technologies Ltd., the company said it is working to satisfy provisions outlined in the European Commission’s recent conditional approval, which requires Western Digital to divest its business in 3.5-inch hard disk drives, including a production plant and related assets.

The $4.3 billion deal was set to make Western Digital the undisputed leader in drives but recent troubles have cast doubt on that likelihood.

The acquisition is expected to close by March, according to executives.

The company expects acquisition related charges in the December quarter of approximately $15 million.