What happens if an error occurs while replacing existing application during dual-bank DFU?

[Woosuk Lee]

As a final step to the legitimate dual bank-based application DFU, the existing application will be replaced by the new application.

What happens if an internal error occurs during the copying/replacing process? How does the bootloader guarantee the integrity?

[Andy]

The bootloader doesn't delete the contents of bank 1 until the contents of bank 0 have been CRC'd. If an error happens, the new application will still be in bank 1, available for copying into bank 0.