On 04.05.20 13:07, Wols Lists wrote:

https://raid.wiki.kernel.org/index.php/Linux_Raid

Is the drive formatted with one big partition? I guess it is. What it
really cares about is that the new partition you are adding is identical
in size to the partition the raid is on.

There are tools that will copy a partition table for you - BEWARE - a
lot of things rely on GUIDs and at least the tools I know of don't reset
them by default - duplicate unique ids are not a good idea :-) So as you
copied this drive using dd DO NOT put it back in the original computer ...

https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm#Adding_a_drive_to_a_mirror

Cheers,
Wol

Thanks, Wol, especially for the hint with the GUIDs, will keep this in 
mind. If ever using it again - maybe in case of a quick temporary 
replacement in the original computer - I will wipe it with zeros before.

The partition layout will be cloned using sfdisk.

Thanks for the wiki links. I read the wiki before asking but it was not 
clear to me how to do it...

Btw, I will stay with mdadm/lvm/ext4 and not switch to btrfs.

Thanks again,
Steffi