NEW YORK (CNNMoney.com) - When Darragh Marmorstein's computer crashed, the freelance wardrobe stylist had to shell out $1,500 - more than the cost of many new computers - to recover her data, which included numerous family photos as well as her professional portfolio.

But she estimates that the cost was much higher when factoring in all the lost opportunities for work she missed because she couldn't send potential clients her files electronically.

The problem was that she hadn't backed up the data.

"I collect tons of photos of my work in order to get other jobs. Everything I had was lost," said Marmorstein. "I had lots of personal family photos that I thought, if I never saw them again, I'd be devastated. Ultimately I recovered it, and I learned a really hard lesson this way."

Of the nine in ten computer users who store personal information on their computers - such as family photos, personal contacts and resumes - about 24 percent report that they have lost personal information in the past month, generally because of system crashes, according to a survey of more than 2,400 users commissioned by software maker Symantec (up $0.36 to $15.73, Research) and conducted by Harris Interactive.

But of those who store personal data on their machines, only 57 percent back up their data. Among users who don't back up their data, just over a third said it hadn't occurred to them to do so, while another 31 percent said it simply wasn't important to them.

Marmorstein said she hadn't thought about the importance of backing up her data until it was too late. Only then did she realize how devastating it would be to lose all her work as well as her family photographs, she said.

"Your heart and soul, your brain - it's all on your computer, and when you lose your hard drive, it's terrible," she said. "I figured, 'This stuff doesn't happen.' Now I tell all my friends that aren't really computer savvy that what they really need to get (is) that external (software backup) program."

Alan Hansen, a senior product manger at Symantec, said the most common cause of lost data is system crashes, followed by viruses. Another common cause is users accidentally deleting their own files; catastrophes such as house fires are another, albeit less common, cause.

Hansen recommends that users purchase an external hard drive, which can be plugged into a computer's USB port, and also install software that will automatically save computer uses' data and perform periodic system backups.

"Only about half the people back up their data, and they don't do a very good job," Hansen said. "It's something they don't think about, or they think it will never happen to them or it's too complicated."

------------------------

AOL fixes e-mail glitch: More here.

How to keep safe in a dangerous digital world: Click here.