May have some corrupted data. With any luck, you are on Windows and may be able to recover it by running chkdsk:

http://www.tekrevue.com/tip/how-to-s...-in-windows-8/

That guide details both fixing errors and scanning/recovering bad sectors. May not need the /r switch (sector scan/recovery)--just the /f switch may suffice (fix errors found in the file structures and such).

Using the /r switch can take a long time, and should really only be necessary if your drive is getting a lot of errors. Windows is notorious for knackering up things in the file system over time, and often needs a little tightening up here and there---especially if the system rarely gets shut down. There are some basic housekeeping rules that need to be run in NTFS from time to time during shutdown/startup.