It can happen very quickly: you empty the Windows Recycle Bin just a bit too quickly, or format a thumb drive unintentionally - and important data is suddenly gone.
Yet hope is not lost, because in most cases the operating system has not actually deleted the file but just released it for overwriting.
"It's similar to a thick book that's had part of the table of contents ripped out. Then you can't find specific pages without a bit of help," says Boi Feddern, an editor at German computing magazine c't.
The files can still be recovered - if you have the right recovery tools. These search through the surface of the hard drive to look for snippets left over to be restored into complete files. Cost-free programs are available on the Internet. "The freeware options are effective enough in many cases and are easy for beginners to use," Feddern says.
One cost-free tool, PC Inspector File Recovery, from Convar, reconstructs files with their original time and date.
Accidentally deleted vacation photos can be retrieved with 'PC Inspector smart recovery', which is intended for storage media for digital cameras. If the problem runs deeper, however, then you'll have to purchase more powerful software.
"If the missing file is a system file or a special file format, then freeware can't get the job done," explains German computer expert Jaroslav Smycek.
Yet there's also a lot of variety among the commercial software as well. The recognition rates can vary greatly, Smycek says.
"Particularly for very large files and special formats like zip or pst, many programs just aren't up to scratch," says Robert Globisch from the online catalogue computeruniverse.net.
One product that has drawn praise from the experts is the 'EasyRecovery' series from Ontrack. The 'professional' version contains an extensive tool collection for recovery, repair, and diagnosis. The software can recognize up to 290 file formats, the manufacturer claims.
Given the price of around $570, the software is intended for users whose files are truly essential. The scaled-back 'lite' version is available for around $94 but can only restore up to 25 files per search pass. Another tool packet is 'RescueBox' from O&O for around $100. This can be run straight from CD for emergency cases.
To keep the chances of success high, the user should stop writing to the hard drive as soon as possible following the loss of data - which means no installing the recovery tools either. "The danger of overwriting important data fragments is high," Smycek explains.
Unsuccessful rescue attempts can make the damage even worse. Users should therefore first make an identical image of the drive. Further recovery attempts can then be made with the image file at a later time. R-Studio, from Haage & Partner (starting at around $50), offers this kind of functionality.
Recovery tools are also helpless against physical defects in the hardware. If the read/write head on a hard drive is damaged or afflicted with water or flames, then a trip to the data rescue lab may be the only remedy.