Add a 3rd drive to a existing raid1 (not as spare)

[basti <black.fledermaus@arcor.de>]

Hello
I want to add a 3rd drive to a raid 1 array (for disaster backup, the
drive will be connectet once a week).
I dry:

mdadm --add /dev/md1 /dev/sde1

when I fail the drive there is a message

FailSpare event detected on md device /dev/md/1, component device /dev/sde1

How can I add the 3rd drive as "real drive" and not as spare?

Best Regards