I have fitted what I described as he lugs on to the new hard drive, and replaced the tray. Everything went back to together as I expected, but Disk Utility still cannot find the hard disc. I am beginning to wonder whether those are not lugs, and that I was wrong to lever the tray off the old drive with my finger-nails. If they are in reality simply screws holding the drive in position in the tray, I may however imperceptibly to the eye have distorted the tray, and that may be affecting the result.