To explain what recuva does when it deep scans. When the header information(directory entries) are deleted(disk format), there is no way to know where the file starts. So what Recuva does is look for a footprint that begins every type of file. Then it traces all the bytes till it hits a end of file marker. However it would be impossible to have every file type possible. To see this use a hex editor, and about the 8th byte in you will see a discriptor of a file, followed by the footprint(which usually contains screen size, audio, etc). Since Recuva does not have the footprint info for mkv files, it will never find them.
However if you just deleted the file, the header info is still intact in the directory as it only flags the file as deleted. Recuva can modify this info to see this file and recover it.
Again, unless the creators of recuva add the footprint for the mkv files, I am boned with any mkv files I have on that drive. And have learned a very valuable lesson. Make sure you look before clicking yes, and next... LOL
again thanks for such a great and simple product.