I am quite eager to find out what the problem is. The HPG10 manual does suggest that some disks may not meet the necessary requirements (pp. 54-55). I understand that bus power is not the problem in your case and I also gather that you are aware that you can (and must) allow the HPG10 to format the Hard Disk (to Type S format). You have evidently used the HPG10 before. Did you try to just first have your Mac format the Hard Disk (using erase), assuming it does not contain anything you’d like to keep, for no particular reason, really, other than trying to clean the slate, and then hook it up again to the HPG10? This suggestion may well turn out to be useless, of course, but I would certainly give it a shot. Good luck.