The CPU Fan Error MAY be just because your fan-checking routines in BIOS Setup have been set with a minimum Fan speed limit of 600 rpm, and the fan is actually not running that fast. Probably that's because the CPU temp is OK at 595 RPM and it does not need more at low processor load at start-up. That is good. The solution may be simply for you to change (reduce) the setting for min fan speed that the CPU fan checker uses for sending out an alarm - say, 560 rpm.
If that is not possible, there may be a place for you to set the automatic CPU fan speed control system to use a higher minimum fan speed like 650 rpm.
On thermal paste, I agree you rarely need to change it.
Another factor for best cooling is clear airflow paths. Keep cabling neatly bunched up to the side as much as you can, and make sure the areas around the fans (even on the outside of the case) are free of obstructions.
A USB2 device on a USB3 port will work, but only at the USB2 speed.
External hard drives and hard drive enclosures are two types of devices available now in USB3 form. Both are an improvement over USB2, The USB2 interface was slower than a typical HDD speed, but USB3 is much faster. This you will achieve the full speed capability of the HDD (but less than the max speed of USB3). Many common USB devices like mouse, keyboard, printer, etc. don't need to additional speed of USB3. So, if you have both USB2 and USB3 ports on your system, use slower devices on the USB2 ports and reserve USB3 for faster ones.
eSATA is another interface for connecting external devices, most often hard drives in enclosures. It is an enhanced version of the original SATA device interface. Two significant enhancements are ability to use longer signal cables (because signals are done at slightly higher voltages) and support for Hot Swapping. It is faster than USB2 and close to Firewire 400 (aka IEEE1394a) speeds, but not quite as fast as USB3. On the other hand, all three of these interface designs are much faster than HDD's themselves, so using any of them for an external HDD means the actual performance is limited by the HDD speed, and not the interface. For example, I have a SATA HDD mounted in an external enclosure and connected via eSATA. It performance is just about the same as my internal HDD's.
An eSATA port on your system which supports the "port multiplier" feature allows an external device also so equipped to connect several (up to 4? or more?) devices (for example, several HDD's in one external case) to a single eSATA port on the host computer.
When you use two monitors driven by a video card that supports such an arrangement, each monitor often shows you only a portion of the total "screen". Then you place your Windows on whichever monitor you wish. For example, some users would place a game or two in windows on one monitor, and e-mail and a word processor window on another. I am NOT clear, myself, how well Windows handles two simultaneous mice. I suspect that it can be done as long as you don't confuse it by moving both mice at the same time. However, I doubt that would achieve what you want. Windows will only act as if there is ONE mouse in the system, and place only ONE mouse cursor on the screen. No matter which physical mouse you move, it will START to move the cursor from its current position. To get to the other screen, you're still going to have to move the mouse there. You might as well use just one physical mouse to do that.
Later thought: I see my son using two "mice" for a good reason. The second is some special "super-mouse" controller for gamers, and he has that connected as well as a regular mouse. When he's playing games on his main larger screen, he uses that special controller. When he's doing simple things or writing code on the smaller screen, he uses a plain mouse. But still, the two devices control only one on-screen pointer, and he moves it where needed.