3.10.11.5 EEPROM Write Prevents Writing to SPMCSR

Note that an EEPROM write operation blocks all software programming to the Flash. Reading the fuses and lock bits by software is also be prevented during the EEPROM write operation. It is recommended that the user checks the status bit EEPE in the EECR register and verifies that the bit is cleared before writing to the SPMCSR register.