Readynas Ultra rootfs entirely in NAND flash?

There is also an OS partition on each disk, not sure how the ultra manages its flash.

the os resides entirely on the disks except maybe the bootloader.. so the free space in flash is entirely unimportant in this case as you dont have access to the flash anyway.

any 3rd party programs you want to install will be placed in the 4gb os-partition on the raid and/or the data volume.

the nand flash is only used to store the firmware image, which is then installed to the drives upon factory default/initialization. Nothing runs from nand itself.

the device will not run without drives, the OS runs off the system partition, generally like a regular linux system.

Thank you for the answer. From a reliability perspective I would like to run the system from NAND flash, as MTBF for NAND flash is considerably higher than for hard disks.

I can think of better reasons for having firmware execute from some form of flash, but not overall system MTBF which will be heavily driven by the least reliable component of the NAS and that will invariably be the disks which you still have storing your data.

But if that is your decision then afraid no ReadyNAS devices will meet your requirement