The diskette may be damaged and is not usable, or less likely, the drive is defective.
The diskette media may be defective or the wrong type. Replace the media if possible.
The diskette drive and/or controller may be defective. Run a diagnostic to validate both the drive and the controller are functioning correctly.
Several software products that offer an extensive range of hardware diagnostics includes Fix-It Utilities Professional and SystemSuite Professional. Other diagnostic products include Eurosoft's PC Check and Iolo's System Mechanic.
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