I've successfully repaired a couple of Humax recorders by replacing the faulty hard drive, you simply swap the drive and the operating system will format it and sort itself out on power up. You might have to pre format the drive in EXT3 on certain models though. As for recovering the data, it might be worth cloning the old drive to see if the Humax box will recognise it on boot up but failing that you'll need to extract the video files from the old drive to play on a PC using the appropriate software.
Don't send the box to Humax as they won't save the data and the cost will be prohibitive compared with the cost of a new drive or even a new box.