Step 1
I ran the RAID in the degraded mode:
mdadm --assemble --scan
mount /dev/md2 /mnt
cd /mnt
mount -t proc /proc /mnt/proc
mount -t sysfs /sys /mnt/sys
mount -o bind /dev /mnt/dev1
chroot /mnt /bin/bash
mount /dev/md1 /boot
mount -o bind /run /run
mount --rbind /dev /dev
update-initramfs -u
update-grub
grub-install /dev/sda
Step 2
I wrote to Hetzner:
My server «EX40-SSD» must have 2 drives: «2x 240 GB SATA III SSD».
Today I noticed that my operating system stopped seeing one of the drives.
Since the operating system does not see the defective drive, I do not know its serial number.
The serial number of the intact drive: «PHDA4096008Y2403GN».
Step 3
After Hetzner replaced the failed drive, I added the new drive to RAID.
How did I add a new drive to my RAID1 instead of a failed one?