Want to know data transfer rates for streaming video, such as from an external hard drive, or from an NAS or a home network. All I'm asking about is the data transfer speeds for clean video streaming.
That depends entirely on the codecs and their settings. For example the standard values for Blu-ray Discs are:
The bit rate of the audio codec is variable, but should stay under 4 Mbps. That's a total of 22 Mbps for 1080p and 12 Mbps for 720p. Lossy compression may result in much lower bit rates.
In comparison, some transfer speeds1:
Even USB 1.1 could be fast enough for 720p. USB 2.0 and every modern hard drive should be fast enough for 1080p.
With Ethernet, it depends entirely on the speed of the involved components and other network activity. If all components support Fast Ethernet, the transfer speeds should be fast enough for 1080p.
1 For effective transfer speeds, protocol overhead has to be taken into account.
2This is not a standard. In practice, an Ethernet connection labelled as, e.g., Fast Ethernet could have a a different transfer speed.