I have a project where in the finished product (sketch loaded to Arduino and it's running on its own), Arduino needs to regularly read data from a text file stored on a SPI Micro SD storage board (the ones sold on ebay). The problem is that the same file also needs to be regularly updated by a user by using a PC connected to the system via a USB cable. The user to simply use Windows Explorer to browse and open the same text file stored on the SD card and edit/save it with notepad. What is the simplest way to accomplish this? What hardware do I need? Is there a SPI Micro SD storage board that also has a slave USB connector (and its interface hardware) to allow access to the file system on the Mirco SD card with PC's Windows Explorer?
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.