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In the event I did have a drive with windows files on the bad sectors, I would image the drive and run chkdsk to repair any needed windows files. I have imaged a dying drive many times without issues. But, if that's the case, the computer would probably be blue screening or having some other major issues. This CAN be risky if there are needed windows files on those sectors. If there are bad sectors, you can do a force clone and have it ignore the bad sectors. If it is going through an ocassional not wanting to be recognized by BIOS, it can be cloned as long as the drive manages to show up. It depends on how far along the drive is from going bad. Better yet, now is a good time to upgrade to Windows 7. If you have the Product Key then reinstall windows using a OEM Vista Installation Disk.
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Backup Vital Data to USB Drive, or remove the hard drive and use an external 2.5" docking station to extract the data to your new HD once windows is reinstalled. It will only fail under that type of task intensive use. If the drive is going bad, don't try to image it.
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