Personally, I think that what used to come up is Windows Explorer (WE) because there's no reason for Autoplay to come up for an external hard drive. For instance, I just connected my cell phone to a USB port and Vista said it was installing the driver and then reported that it was finished. Neither WE nor Autoplay came up. I think that WE didn't come up because of what I said about Autorun. When I opened WE, the cell phone connection was shown as a drive. It's interesting because the only thing on my cell phone memory card is photographs. Perhaps if AutoRun was enabled, AutoPlay would have opened.
Of course, I could be wrong.