That's the bare minimum for today's drives. Western Digital advertises 12x for the low-power 5400rpm "Green" drives, which are found in a number of cable and satellite DVRs. Modern hard drives are never the bottleneck in a DVR, because each MPEG HD stream consumes a maximum of 19.4Mbit/s (2.425 MB/s), and usually much less.
The bottleneck in a DVR is the CPU and memory. Unlike the hard drive, those components must deal with uncompressed HD streams, which require up to 50 times the bandwidth. The BCM7400 in the Moxi is spec'd to buffer and record up to four HD channels, while it decodes two different HD channels for display. Dish Network uses the same chip in their ViP722K and [upcoming] ViP922 DVRs to output different channels to two separate TVs from the same box.
The triple-tuner Moxi can record and independently buffer three HD channels simultaneously, plus one more analog channel when the USB dongle is connected. While doing that, it can also playback a HD recording while streaming two other HD recordings to MoxiMate extenders elsewhere in the home.