CTR2 uses the built-in SD card slot on the Teensy 4.1 development board to store setup data, audio Tx buffers, off-air recordings, and voice announcement files. The SD card slot on the Teensy Audio Adapter board is not used.
While it is possible to just plug in an SD card and run CTR2, this is not recommended. CTR2 will create the necessary folders and default files, but it cannot create the .WAV files used for frequency and mode voice announcements.
For full functionality, download, extract, and copy the folders in the CTR2_Teensy_SD_Default_Files_xx-xx-xx.zip file to a blank SD then insert this card into the SD card slot on the Teensy 4.1 development board. The SD card can be up to 256 GB but a 16 GB card is usually more than sufficient. These folders provide the basic folder structure for CTR2.