Cannot connect to SD card with MD_MIDIFile on Teensy 4.1

Question

I'm trying to use the MD_MIDIFile library from majicdesigns to read MIDI files from an SD card and translate them to human understandable language, on a Teensy 4.1 board (because I need the RAM). But when I do, it won't recognize and/or connect to the SD card. I'm using PlatformIO on VSCode.

I took the MD_MIDIFile_Loop example from the library to try the basics of the library. It first gave errors stating that File is not defined. I discovered this has to do with SD_FAT_TYPE in MD_MIDIFile.h, but I tried every possible value (0 - 3) and they all won't work. The only code I changed in MD_MIDIFile_Loop is on line 75 to if (!SD.begin(254)), to make it work with the Teensy (the normal code is for Arduino). It will get stuck in that if statement, indicating that the SD card is not found. For clarity: I didn't get any error messages, it just doesn't work. I got 254 from the SD.h library from Teensy, which is defined as SD_BUILT_IN. Without the MD_MIDIFile library, theSD.h library works perfectly, butSDfat.h doesn't. This question kinda is what I need, but I don't know where to put the file using PlatformIO.