1) Is it necessary to have a background on the older SD library (bundled with IDE) to start working with the SDfat library.
Is there a fundamental difference between them?
3) How do I set the Chip Select(CS) pin permanently.
4) Are there any suggestions on working with the library
Between SDBaseFile and SDFile? SDBaseFile derives from FatFile. SDFile derives from PrintFile which derives from FatFile and Print. So, fundamentally? No.
I'm not sure what you mean by permanently
Starting with realistic expectations, just do it.
As i mentioned earlier there are a few examples with CS set to pin#10 and other to pin#4.
Does a write to an SD card pushed to background (task) and the arduino can resume the normal program meanwhile? Or does the program execution halt during the writing operation.