After all my sorrow with the first build of a mirror I had to restore the factory defaults and re-image before MYVOLUME1 showed up again.
I have used the Asus since then intensively.
Now I have another go, deleted and created/formatted a foreign disk. But the Asus refuses to build the mirror this time:

Add Mirrors/Spares Result

OPERATION FAILED!

Error while adding mirrors to pool `MYVOLUME1': Failed trying to add mirrors to pool.

??? What pool, where, why?
I tried this 3 times with the same result. The message pops up very quickly after confirming to create the mirror
Hope someone knows what to do next...

Greets,
Marc