How to do incremental vertical expansion in FlexRAID mode using SSH commands

I have verified that it is possible to add multiple drives at once. But you must start with at least a 2-drive RAID1 when you do. So, adding partition three of two drives in slots 3 and 4 to an existing two-drive md127 RAID1 would require:

# mdadm /dev/md127 --grow --level=5 --raid-devices=4 --add /dev/sdc3 /dev/sdd3

I have also found that is is possible to grow a single-drive JBOB (really 1-drive RAID1) to a 2-drive RAID0. But, you have to do it in two steps. The following grows a single drive JBOD by adding partition three of a drive in slot 2:

# mdadm /dev/md127 --grow --level=0
# mdadm /dev/md127 --grow --raid-devices=2 --add /dev/sdb3

This will temporarilly change it to RAID4, do a sync, then change it to the desired RAID0.

If you are adding a drive (like the expansions above, or for any reason) rather than replacing one, it is difficult to set up the proper OS and swap partitions (fdisk doesn't like to start where md0 does on the ReadyNAS) and also to make sure that those partitions get synced with the others that make up the OS and swap RAIDs. The best way I found to do that is to use the GUI to tell the NAS to create a new volume on that drive, then DESTROY the volume (which you can do before it completes, if desired). You can then use cat /proc/mdstat to see the md0 and md1 syncs complete and then use fdisk to create the partition(s) you intend to add.

This ends my experiments. I've not touched on RAID6, RAID10, or RAID50, but I'm sure the process is pretty much the same, you just have to insure you have the correct minimum number of drives. If it refuses to do a conversion, you may need to break it into two steps, like for the RAID0 expansion above.