If I opt for Additional 2.5" 2 TB 5400RPM SATA Hard Drive, the battery 4 cells 64Whr battery should be selected.
This is probably because the larger battery takes up the space where a 2.5" HDD can be installed.
Does Using both SSD & SATA Hard drive impacts Laptop Performance?
There is no surprise here, things that run on the SSD run at the ...
It doesnt affect performance, as long as nothing is on the hard drive. If you actively use the hard drive, the app that uses the hard drive slows down to hard drive speed. For some tasks (like rendering a video), you can write to the hard drive without performance issues because the bottleneck is somewhere else (cpu)
That device won't work on 3.5" HDD drives because USB-A doesn't have the 12v rail those drives need.
USB-C has various ways of delivering higher power, but not USB-A.
The solution is an external power supply or something with USB-C with a PD specification.