The X1 DVR is nice, but it still has a few glitches. Sometimes the response times to your requests are a bit long and occasionally it will not respond at all, especially when working with the list of recordings. I was having more trouble until recently when a user here advised rebooting once a week. That seems to make it work better.
I don't know the size of the hard drive, but it fills up pretty fast. I have to make sure I delete anything I don't want so that it won't delete stuff I do want to keep. When it gets close to getting full, it starts deleting the older files.
I think four shows is the maximum for recording. I know it will do four, but I haven't tried five.
All of the "On Demand" programs are on the cloud and they download quickly and play smoothly.
I don't know of any way to attach an external hard drive to it.