Finally got some time here and yeah, had no luck here! =Z
I could get the SD working using the Tegra Explorer (for some reason, just by entering on it made the SD recognizable for the "boot.dat" issue but, the problem got into the main games later on
Got some games into the "sxos" folder as usual, and, when accessing it on the Sxos image album menu, it just locks it up, not getting anywhere else from there, forcing me to reset the switch
Maybe it´s a problem with the card´s size (256 GB), don´t know if that´s too big for sxos (which is curious, cause it reads the external drives just fine)