It pops up an error message as follow when you access the RAW drive by double-clicking it.
Invalid media type reading drive. Abort, Retry, Fail?
Windows may ask if you want to format the drive, such as a warning reads "The disk in drive (X) is not formatted. Do you want to format it now?".
File names contain "weird" characters.
"Sector not found" messages.
Also a CHKDSK tool will complain that it is unable to perform a RAW file system recovery. The type of the file system is RAW. CHKDSK is not available for RAW drives.
Sometimes people call it raw drive or raw file system. The properties of the drive often shows 0 bytes while there were important files, photos stored there before (Right click the drive and select "Properties") and the file system shows RAW. RAW file system is not really a type of file system but presents a problem of your drive or partition. We know often used file system like NTFS, FAT16/FAT32 or EXT2/EXT3. So what is RAW file system? Just a system error that makes your partition become RAW and you cannot access files there without data recovery software.