The pin got bent when I attached the adapter yesterday, and thus it was not in the corresponding hole in the adapter. I was unaware of the situation and installed it. The drive spun up but was not recognized by the BIOS.
However, I'd had problems like that with this drive before -- sometimes it would play with a motherboard but often it didn't The drive or the adapter may have already been damaged, but I was far from sure. Installing and uninstalling it in the mATX case is a bit of an ordeal, so I don't want to keep experimenting if the loss of the pin spells the end for all practical purposes.
So, it's time to remove it from the arsenal. Thanks to all for the help in making the decision.