Might be the missing piece, to get it to work on the Zyxel.
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Removing the /dev/md127 was another task, but this time, google was my friend!
Normally you have to make it faulty before you can remove it. But that only counts for real raids. But i finally found the right command
mdadm --manage /dev/md127 --stop 
I removed the partition, made a new ext4 and a format. Now i'm putting back part of the files i had on my linux box.
Now that i know the procedure i hope the other disc will go smoothly! (don't say it out loud!!)
If you want a combination of stability and performance then xfs is your best bet for a NAS filesystem.
I already formatted on ext4. And i'm almost done with copying 2TB...
But i will keep that in mind. What are the cons and pro's of xfs?
Greetings from 2024. 
This nugget alone just saved me.
Thank you, kindly.