It may happen when you take out the storage device (memory card) not using "safely remove" feature, the next time you connect the storage device you can receive a message "You need to format the disk in drive X: before you can use it".
Typically this means that the filesystem on the drive became RAW. Obviously, initial file system was corrupted because the data stored in the buffer during disconnection of the drive was lost due to inappropriate disconnection.
To get rid of the RAW file system issue you need just to format the drive, however, be aware that your data would be gone forever according to the format type you apply. If the data is important you need to recover data first and after that do the format procedure. To recover data you can just use some data recovery program.
