Definition
NAND flash is a type of non-volatile solid-state memory used to store data that must survive a power cycle. On many ASIC control boards it holds the entire firmware stack — bootloader, Linux kernel, and root filesystem — which the Zynq SoC reads at power-up. When a miner "loads firmware" on boot, it is reading from on-board NAND (or its successor, eMMC).
Why NAND failures brick miners
NAND is rewritten in blocks, and a firmware update erases and reprograms those blocks in place. If power is lost mid-write — the most common cause of a bricked control board in our shop — the NAND is left in a partially written, unbootable state. The SoC then halts at the bootloader or loops endlessly, with no working filesystem to load.
Recovery paths
Recovery depends on the board. Some controllers expose an SD-card slot or recovery mode that lets you reflash a clean image; eMMC-based boards such as those on the S19 XP and S21 typically have no SD socket, so recovery means reflashing the storage directly or accessing the serial UART console to intervene at boot. Always flash from a stable power source and a known-good image.
NAND wears out after many program/erase cycles, which is one reason firmware should not be reflashed needlessly. For the boot interface that reads it, see Zynq SoC; for the modern replacement, see eMMC.
In Simple Terms
NAND flash is a type of non-volatile solid-state memory used to store data that must survive a power cycle. On many ASIC control boards it…