Raid1 with disk fails not boot

[Jose Legido <jose@legido.com>]

Hello!
I have a software RAID1 with 2 discs. When I get out one disk, the
system not boot directly, I have to do mannually actions:

Loading, please wait...
Gave up waiting for root device. Common problems:
 - Boot args (cat /proc/cmdline)
   - Check rootdelay: (did the system wait long enough?)
   - Check root= (did the system wait for the right device?)
 - Missing modules (cat /proc/modules: ls /dev)
ALERT! /dev/disk/bu-uuid/c511cf66-d987-477e-96fe-a5fc350d1bB4 does not exist.
Dropping to a shell!
modprobe: module ehci-orion not found  in modules.dep

BusyBox v1.22.1 (Debian 1:1.22.0-9+deb8u1) built-in shell (ash)
Enter 'help' for a list at buil-in commands.

bin/sh: can't access tty; job control turned off
(initramfs)

I look the raid:
(initramfs) cat /proc/mdstat

md0: inactive sda2[0](S)
        8090624 blocks super 1.2
unused devices: <none>

(initramfs) mdadm --detail /dev/md0
/dev/md0:
  Version: 1.2
  Raid Level: raid0
  Total Devices: 1
  Persistence: Superblock is persistent

  State: inactive

  Name: debian:0
  UUID: 688aa204:88cf0db8:4a94610d:a3e3dc03
  Events: 108

  Number  Major  Minor  RaidDevice
      -            8        2            -              /dev/sda2


If I restart the RAID, the servers run:

(initramfs) mdadm --stop /dev/md0
mdadm: stopped /dev/md0

(initramfs) mdadm --assemble /dev/md0
mdadm: /dev/md0 has been started with 1 drive (out of 2).

The server boots:

# cat /proc/mdstat
Personalities: [raid1]
md0: active raid1 sda[0]
         8090624 blocks super 1.2 [2/1] [U_]
unused devices: <none>


# mdadm --detail /dev/md0
/dev/md0:

Version: 1.2
Creation Time: Wed Aug 12 10:11:58 2015
Raid Level: raid1
Array Size: 8030524 (7.72 GiB 8.28 GB)
Used Dev Size: 8030524 (7.72 GiB 8.28 GB)
Raid Devices: 2
Total Devices : 1
Persistence: Superblock is persistent

update T1me : Thu Aug 13 05:17:41 2015
State:  clean, degraded
Active Devices: 1
Working Devices: 1
Failed Deviices: 0
Spare Devices: 0

Name : debian:0 (local to host debian)
UUID: 688aa204:88cf0dfb8:4a94610d:a3a3dc03
Events : 154

Number Major Minor RaidDevice State
    0          8        2         0           active sync   /dev/sda2
    2          0         0        2            removed

I restart the server, add the disk and rebuild RAID:
# mdadm /dev/md0 -a /dev/sdb2

And it  works again

How can I boot without mannually action?

Thanks!