So I was using my Mac with my external hard drive connected via USB.
I moved about 10 GB of data to it (via drag and drop while holding down the Command key to move the files rather than to copy them). They moved to the drive all right, but as I was having some issues and the Finder crashed after the transfer, I was unable to eject the volume and later everything froze so I had to do a hard restart (hold the power button).
When I remounted the volume (plugged the external hard drive back in) it no longer had any of the files which I moved onto it.
As it was a lot of data, how can I recover these files?