Copy data from removed jbod disks

is this normal that I could read the disk1 in an usb-hddrack but couldn't read the disk2 in the same rack?

My first reaction is no, according to your fdisk dump, the disks are functionally equal. But according to your question you had this problem, which makes me wonder how you read that disk. There is a difference. The first disk has a first partition of type 8, the second one a type 83. Don't know why. But if you used software which simply searched for the first recognizable filesystem, it would skip that 8, and end in the 2nd data partition. While on the second disk it would find the 83, which is a normal, mountable partition containing a few firmware files.

Is there a way to create new volumes in the NSA326 later which are independent and could be read with no hassle in case of NAS failure?

Create 1 volume per disk. On a ZyXEL that will still be a raid array (I think a single disk raid1, but it could also be a single disk linear, like you have now), in that case you always need only one disk to mount the filesystem.

More hassle free in not possible.