Adafruit Trinket M0 persistent storage?
The M0 boards use a SAMD chip. It has no EEPROM. To store data without additional hardware like external EEPROM or SD card, you can use the flash memory of the SAMD. But it has a drawback of limited count of writes to flash memory. 

An EEPROM emulation library exits. It uses the flash memory, so the drawback of limited writes, applies. I used it, it works. The library is almost official. The author is Arduino's coworker.