There are several reasons for missing files and the following section will suggest approaches that can be tried to recover all files
The subdirectory for the files has become separated
If recovery is being performed using the system directory structure then a corrupt directory entry can cause all files under that directory to be lost. The solution for NTFS is to recover from File Entries.This may mean that files are stored in dummy directories, but they will be found. For a FAT disk, use the Recover from directory stubs option.
A complete partition is lost
If the partition table has been corrupted, then it is possible for a complete partition to be lost. The solution is to use the Partition function and Analyse Partitions, and then Scan for previous partitions.
On a Unix disk, different file systems may be installed, but typically a UNix disk dos not always have a DOS compatible boot sector with partition tables. Again, a partition anaysis and scan for previous partitions will find any existing partitions.