Even a standard quality 5400RPM drive is going to be able to max out the USB2.0. transfer speed. But in this use case, thats still more than required…
The Highest quality setting available is 10Mbps.
The max simultaneous streams is rated to be 6. And a 4 tuner could in theory be recording 4 different things at the same time… Luckily this makes for some easy math with 10 video streams set at 10Mbps…
Now I am not sure if that 10 quality setting is strictly video but even if you factor in another 500K each for some insanely high quality audio, you are still only at 105Mbps.
While USB2.0 is rated at 480, we know thats not possible in the real world, 200 ~ 240 is more common, which is still DOUBLE what the max load would be for the Tablo, set at max quality with 6 streams of pre-recorded content and an additional 4 tuners recording 4 other streams.
So as far as data transfer any way you look at it 5400RPM and USB2.0 is plenty.
Now, there are 2 areas that (IMO) you might see some improvement with a higher speed / higher rated Hdd.
First, and where I think it makes the most compelling argument, is duty cycle ratings. While transfer speed is not an issue, you are correct that the standard portable Hdd is not typically rated for continual use, for this reason alone, I would second the argument that a desktop, continuous duty cycle drive SHOULD give you longer / more reliable use.
I also prefer the desktop Hdd because I prefer to not power it from the Tablo. It has been beaten to death that Tablo runs on the warm side (and we have seen a decent number of power supplies die), so while I have never believed mine was TOO hot, even when I had a USB powered drive, the idea of taking some load off the Tablo helps me sleep at night.
The second thought isn’t so much about sustained transfer speed but rather initial load time. It does take a few seconds for Tablo to first wake up, tune to a channel, spin up the Hdd and start encoding the stream for playback. So while 7200RPM is probably not needed for sustained use, does it spin up and start taking data faster?
In this area a few have tried solid state drives and reported an improvement. I have been tempted to try them too but the price is still a bit too high for the large capacity I would need and I worry the duty cycles of SDD isn’t where it needs to be for this type of use.