When you access this drive, you will get an error message "The disk in drive is not formatted. Do you want to format it now?". In this situation, the file system is shown as "RAW" when we check the disk property. If we click "No", the drive cannot be accessed. On contrary, if we click "Yes", we will lose all of the files stored on this drive.
A corrupt or missing boot sector will cause such kind of problem.To fix the boot sector, you may refer to How to Fix the Boot Sector
If it still can not be solved, you have to recover all the data out and then format the partition so that you can enter the partition.
How to do Raw Recovery or Recover Raw Drive