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Recently I bought an Arduino M0 board and I'm using it in one of my projects, which logs GPS data into an SD card.
Can an Arduino M0 be programmed as a USB SD Card reader? So that I don't have to remove the SD card from the system and use the same USB port from M0 to see the contents of the SD card directly on my computer.
Yes, in at least some way, but what is realizable and best depends on a few things.
A traditional "card reader" is a USB Mass Storage device which basically exports the logical blocks of the card to the USB host computer. While this is active, nothing on the embedded system may interact with the card. The embedded system will need a USB device port which can operate in this mode, suitable software, and whatever data is recorded on the card will need to be a in a filesystem (FAT or whatever) that the host PC can read, and a data format in the files that existing or custom software for that PC can make sense of.
A different option would be to still store data on the card in a way that would be PC-compatible if the card were removed and placed in a PC, but not to be a USB Mass Storage device. Instead, you might use the existing quasi-serial interface that is the typical role of an Arduino-type board's USB port, to send or summarize the data, either directly or by interpreting it first. This could either flow into a simple terminal program, or into something custom. Typically this means less work on the Arduino side, more but more work on the PC side. Additionally, since there is never a direct export of the card's block, the embedded system continues to have access to the card the whole time, rather than having to surrender control of the card to the PC operating system as USB Mass Storage requires.
A variation of the above could leverage the fact that if only the Arduino ever reads the card for its own purposes or to report the data to a PC, it doesn't necessarily need to store things in a PC compatible file system, or even in a file system at all.
