Curiously, having refused to format the disk twice in the last week, when I turned the machine on tonight the Hummy told me that the disk needed to be formatted in order to use the recording functions. It then proceeded to format it normally.
I guess my attempts to format last night had indeed wiped the partition table and removed whatever was preventing the formatter from working.
All's well that ends well. Boot up is now notably quicker. It remains to be seen whether some read errors I was getting (when the machine was recording 2 HD streams and playing one in chase-play) have now been eliminated.
AF123 - it's not a replacement disk, it's the original. That's why I was so puzzled.