AVR Freaks

what BYTE pdrv is.?. BYTE is not a data type.

BYTE is defined as "typedef unsigned char    BYTE;" in integer.h, lots of other types are also used there.

BYTE pdrv is an "unsigned char" (or uint8_t ) containing the drive number as it shows above.

How is that ever going to work?

Above all have you waded through the thread in Tutorial Forum about FatFs. Sure it is many pages but there's a lot of info. 

As you'll discover there and also in more recent threads (ones like this occur once every month to three months) the main issue is what happened to FatFs (for AVR) around the 2015 (was it?) mark. The ffexample.zip file used to have a mega64 example that used the SPI so mmc_avr.c had full working SPI code. Sure it was specific to the m64 layout but was easy to adapt. Then the example was changed (to mega1284p I believe). When this happened the author (not sure if it was Chan himself or a contributor) chose to use the 1284 USART in SPI mode (mmc_uart.c) but for whatever reason they also changed the SPI file to. To make into more of a "template" they cut out great swaths of the existing code and left a "to be filled" comment instead. So it's now much more difficult to use SPI "out of the box". One way (assuming a "modern" AVR) is to use USART in SPI mode, sure that prevents it being used as a UART but it is true that USART/SPI is just marginally more efficient than real SPI because of the additional buffering it provides. If you plan to use the SPI peripheral the way to see a full worked example is to take a trip back in time, get one of the older versions and use the mmc_avr.c that has the full mega64 example code.

As to your current issue. If you get really stuck zip up your Studio 7 project and post it as an attachment and I'll tell you what's wrong and how to fix it. (this may have to wait until I finally return to work next Monday in fact) 

seems also to be some limited experience with C programming?

The problem causing the error in the OP is a C programming issue: you seem to have put a prototype for disk_status() in main.c when it already has a prototype in diskio.h - and those prototypes don't match.

Another C problem in the code posted in #1:

/*
 * Test_FATFSLibrary_Rev2.c
 *
 * Created: 1/5/2022 3:38:28 PM
 * Author : Houdini
 */ 

#include <avr/io.h>
#include <avr/interrupt.h>
#include "diskio.h"
#include "ff.h"
#include "uart.h"

DSTATUS	disk_status(2);         <<< can't have function calls here
DSTATUS	disk_initialize(2);

int main(void)
{

The highlighted function calls there are outside any function - that's not allowed in C.

That's probably a good idea - as the whole point of Arduino is to take care of the arcane detail, and let you focus on what you're actually trying to achieve.

But it is all open source - so you can still dig into it to see what's going on if you want to.

In fact, it seems the Arduino library doesn't use FatFs:

EDIT

It seems Arduino's link is out-of-date - the source is now at: https://github.com/greiman/SdFat

I can't believe I'm actually going to say this but for the very first time in my life I think I actually agree with something Simonetta has said.

Given your experience background Arduino probably IS the way to approach SD/MMC. FatFs is not a beginner lib, it is for seasoned professionals who can adapt it to their needs and understand what is provided and what the user's responsibilities are. If you don't have the experience then the very clever thing about Arduino is that it hides many levels of complexity from the end user and provides a top level, simple interface anyone should be able to understand so you can get on and use FAT on SD/MMC without requiring detailed knowledge of the lower layers. FatFs achieves this to some extent but not to the extent that Arduino does. 

So if you just want a project with some filing but don't want to get bogged down in the nitty-gritty detail of how to do that Arduino should isolate you perfectly.

Another advantage of Arduino is that it's so widely used and supported you should easily find examples trying to achieve pretty much whatever it is you want to do with FAT/SD/MMC.