External hard disk drive become raw file system error

External hard disk drive is raw file system when using chkdsk? Generally speaking, we can not open it when the external hard disk drive becomes raw file system. We need to format the external hard disk drive if we want to keep using it. But the problem is we will lose files saved on the external hard disk drive at the previous time if it is formatted. As a matter of fact, we lose them when the external hard disk drive is raw.