It's the illico 8300HD model. Its about 2 and half years old. Its all original with no external hard drive. I formatted and did the the reboot. The HD signal seems fine, no problems watching live. It's only on playback and recording.
My guess is that the iHDD can handle the lower bitrate of the SD recording, but not the HD recording. You could try replacing the iHDD, especially if there's nothing of consequence on it. Here's a thread on the topic - it's Rogers, but probably applies. You can get a 320 GB PATA/IDE drive for about $100. 500 GB are hard to find. 160s are a bit cheaper.