Add a drive to raid1

First get information about the raid array:
```
cat /proc/mdstat
```
Then get information about the drive you’re going to add
```
fdisk -l /dev/DISK_TO_ADD
```
If there are any partitions, delete them and create a one new primary partition with type fd – Auto RAID Add the prepared drive to the array:
```
mdadm --add  /dev/RAID /dev/DISK_TO_ADD
```

And expand it:

mdadm --grow /dev/RAID --raid-devices= N+1
# N - the number of drives in the raid array before the expansion

Seat comfortable and watch the raid being rebuilded:
```
watch cat /proc/mdstat
```