This message appears when the File System is corrupt and the OS does not understand how to read the data. This is known as RAW format.
Why does it happen?
Usually due to unsafe disconnections ("unplugging the cable"), power outages, or bad sectors right in the Boot Sector or the Master File Table (MFT).
Safe Solution
Never accept formatting. Use recovery software that supports RAW reading (Raw Recovery) to extract files to another drive. Once the data is saved, then you can attempt to format or repair the original drive, although this often indicates that the device is no longer reliable for storing important information.
