SMC fan control reports 1800rpm minimum fan speed. This is with an upgraded SSD and a Dual-DVI -> 30" Cinema Display (using discrete 6630M GPU.)
I can confirm that System Information reports a 6 Gigabit link. My SSD is only SATA II (3 Gigabit) so I am unable to test throughput.
I also have a flat cable from a 2010 mini server to test the second HD.
No. See the note in the guide, it is soldered to the motherboard.
There is no separate temp sensor, just the single flat cable from motherboard to the SATA hard drive (unless the temp sensor wiring is included in a custom SATA port on the 500GB hard drive. With an SSD in place, my fan is noticeably running, but I am not sure whether it is due to heat from the SSD/GPU/CPU combination.)
The same socket exists on the 6630M version (I put an SSD in my i5/2.5/500 prior to this take apart.)
What is the empty socket located about an inch above the CPU and GPU on the motherboard (same side as the SATA sockets)?