You've securely deleted a file, but you can still see the Preview or Header information:
This usually occurs when the file you've securely deleted has already been overwritten with another file. Examine the Preview to determine if the file is really the one you securely deleted. The Header tab, which displays the first 128 bytes of the file, can be misleading, since the contents of the Header tab may not change if the first cluster has already been overwritten with another file.
Some files that have been securely deleted are marked as recoverable:
If you have used Recuva (or any other similar app) to erase files, the files may still be marked as being recoverable, but they are not. The MFT entry may still exist, but the contents of the file are unreadable.
You receive an "Not overwritten - More data is available." message when attempting to secure delete a file:
This notice indicates that the drive or folder that the file was located in are compressed or encrypted at a file system level and, as such, were 'mixed up' with other files. As such, it is not safe to secure delete these files, as this process should only be performed on uncompressed, unencrypted file systems. You can find out more here and here.
I want to recover a file that has been securely deleted or overwritten:
Unfortunately, this isn't possible. Once it's gone, it's gone.