As flood waters filled their basement, Larry and Nancy MacLennan hastily moved their computer to the first floor before evacuating. But the water continued to rise, eventually filling most of the two-story house and submerging the computer for hours.

For the next several days the family worried about the damage to their Minnesota City, Minn., house. When they remembered that the computer held thousands of photos, including some about 70 years old, the MacLennans feared those precious files were lost forever.

But their daughter, 35-year-old Jenna MacLennan, had heard that data-recovery firms now sometimes find data on extremely damaged hard drives. Within days, engineers had recovered all the MacLennans' files.

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"We were extremely happy about that," said Jenna MacLennan, an account manager for an electronic-equipment manufacturer. "With the water, the mud, everything, we just didn't know what kind of corrosion or damage might have occurred."

Hard drives typically fail when mechanical parts wear out, but the drives tend to be remarkably resilient to external elements such as flood water, said Richard M. Smith, an Internet security and privacy consultant at Boston Software Forensics.

"If you look at a hard drive, it's hermetically sealed," Smith said. "In most cases water wouldn't get into the drive itself."

Owners of flood- or fire-damaged computers typically assume their digital tax forms, photos and passwords are unrecoverable. Not necessarily, computer experts say, noting that at least some data can be recovered from virtually any faulty or damaged storage device. And as the computer industry has grown, so has the number of companies doing that restoration work.

"We've done data recovery on a laptop that was dropped from a helicopter, on a laptop that had been submerged in the ocean for a year," said Todd Johnson, vice president of operations at Kroll Ontrack Inc., whose engineers helped the MacLennans. "One time there were even bullet holes in the hard drives."

Kroll Ontrack is a division of New York-based Kroll Inc., a risk-consulting company whose technology operations announced second-quarter revenue in August of $141 million.

As a service to victims of last month's floods in Wisconsin, Illinois, Minnesota, Ohio and Oklahoma, Kroll Ontrack is waiving some costs and charging them a flat recovery fee of $850, with 10 percent to be donated to the Red Cross.

The 20-year-old company, based in Eden Prairie, Minn., is one of several offering similar services and prices, including SalvageData Recovery Lab Inc. in Stamford, Conn., and First Advantage Data Recovery Services in Irving, Texas.

The companies charge from $400 to $2,500 for a standard recovery, with the price varying depending on several factors including the proportion of data that can be recovered.

Data-recovery companies use proprietary methods to recover data, pulling files into their own environment, where engineers can determine which are salvageable. The recovery process involves digging below the operating system, Johnson said.

Data can be salvaged from Windows-based computers and Apple Inc.'s Macs, and even from fully loaded iPods or cell phones. Engineers then ship the files back on CDs, DVDs or on a new hard drive.

Typical computer users know they should back up their data, Johnson said, but many keep their backup files so close to their computers that secondary files are destroyed at the same time as the computer. He recommends that backups be kept at a distance, perhaps even in a safe-deposit box at a bank.

That experts can recover data from hard drives damaged by water, fire or even a sledgehammer is a mixed blessing. Sometimes a person disposing of an old computer actually wants the hard drive destroyed to thwart would-be hackers looking for private information. So how can one be sure the hard drive is rendered permanently inaccessible?

Some experts suggest running a data-erasing program that repeatedly overwrites information with ones and zeros. Others suggest keeping the hard drive and disposing of the rest of the computer. The most extreme option would be to physically shred the hard drive and dispose of pieces in multiple locations.

As Jenna MacLennan looks at recovered digital photos of her grandmother and grandmother's parents, she says the data recovery was a bright spot in a tragedy that left her parents' home as a roof balanced on stripped two-by-fours.

"When you're able to recover your history, your photographs, there's a sense of gaining back something that's yours," she said. "It's something you can look at as good amongst everything else, that your memories aren't all gone."