It may happen when you disconnect the external hard drive (memory card) not using "safely remove hardware" option, the next time you connect the drive Windows will say "Do you want to format it"?
In most cases such a behavior means that you have a RAW filesystem. Obviously, initial file system was corrupted because those data that was in the buffer during disconnection of the storage device was lost due to such a disconnection.
You can easily solve the problem by just formatting the drive - but... the data would be gone away irreversibly. If the data is important you should retrieve data first and after that perform the format procedure. Data recovery in case of RAW filesystem is similar to unformat but typically more effective. It is very easy to do - just try any data recovery utility.
