The file directory is either corrupted or unreadable means that Windows is unable to access the file system data on the hard drive in question. It may happen as a result of sudden power outage, faulty removal of the USB drive, bad sectors or disk corruption. Trying to check the drive using other computer and conducting initial diagnostics might give some results. It would be unwise to format the drive if it contains any files. If your files became inaccessible due to disk corruption, you may try Notchox as the tool for Corrupted File Recovery and so on.