Arduino to read file data from SD storage board, and PC user to update same file via usb cable as needed
Arduino needs to regularly read data from a text file stored on a SPI Micro SD [...]
  regularly updated by a user by using a PC connected to the system via a USB 


The Micro sd cannot be used by 2 independent systems simultanously. You can either use it from the Arduino or from the PC as USB Mass storage but not both at the same time.

Simplest approach from hardware perspective would be a terminal interface (via UART) from the PC (using putty for example) and do all the editing in arduino software, which talks to the SD card via SPI.