Some googling brought me this: Nexx WT3020 - problem with mdadm
Unfortunately, I can't find the kernel config of the current release. (Which version do you use?)
Can you mount any blockdevice? USB stick, or something like that? Can you post the output of
mdadm --examine /dev/sda2
On your Pi, (assuming sda is one of your ZyXEL disks). As you have single disk volumes, it is possible to mount them without assembling the array, and so without mdadm, by using a loop device. But you'll have to know the offset.