Selling a used laptop requires more than just a quick format. Simply deleting files or reinstalling the operating system only removes the directory references, leaving the actual data physically stored on the hard drive (HDD) or solid-state drive (SSD). This means that widely available data recovery software can easily restore your supposedly deleted files.
To truly protect your personal information, you need to overwrite the entire drive with random data—a process often called "wiping" or "secure erase." For HDDs, tools like DBAN (Darik's Boot and Nuke) can perform multiple passes of overwriting. For SSDs, use the manufacturer's secure erase utility or built-in commands like ATA Secure Erase, which triggers the drive's internal wipe mechanism. On Windows, the cipher /w:C: command can overwrite free space, but it only affects already-deleted files; existing files remain untouched.
After securely wiping the drive, reinstall the operating system fresh. This ensures no recoverable traces of your data remain, giving the next owner a clean slate and you peace of mind.