Now I redid the creation and...
root@cinderella:~# mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sdc /dev/sdd
mdadm: partition table exists on /dev/sdc
mdadm: partition table exists on /dev/sdc but will be lost or
meaningless after creating array
mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device. If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
mdadm: partition table exists on /dev/sdd
mdadm: partition table exists on /dev/sdd but will be lost or
meaningless after creating array
mdadm: size set to 1953382464K
mdadm: automatically enabling write-intent bitmap on large array
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
root@cinderella:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sdd[1] sdc[0]
1953382464 blocks super 1.2 [2/2] [UU]
[>....................] resync = 0.1% (2260544/1953382464) finish=230.1min speed=141284K/sec
bitmap: 15/15 pages [60KB], 65536KB chunk
unused devices: <none>
root@cinderella:~# mount -v /dev/md0 /mnt/md0
mount: /dev/md0 mounted on /mnt/md0.
root@cinderella:~# ls -la /mnt/md0
total 36
drwxr-xr-x 6 root root 4096 Feb 18 22:40 .
drwxr-xr-x 4 root root 4096 Feb 18 22:34 ..
drwxr-xr-x 7 mauro mauro 4096 Feb 23 2018 etherpad
drwx------ 2 root root 16384 Feb 18 11:10 lost+found
drwxr-xr-x 16 mauro mauro 4096 Nov 10 11:42 mauro
drwxr-xr-x 127 mcon mcon 4096 Feb 19 15:09 mcon
root@cinderella:~#
Any idea?