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Ubuntu Server 14.04 - RAID5 created with mdadm disappears after reboot

This is my first question on superuser, so if I forgot to mention something, please ask.

I'm trying to set up a home server which will be used as file server and media server. I installed Ubuntu Server 14.04 and now I'm trying to set up a Raid5 consisting of a total of 5 disks, using mdadm. After the raid has been created, I am able to use it and I can also access the Raid from other PCs. After rebooting the server, the Raid does not show up anymore. I have also not been able to assemble the raid.

I have done the following steps:

Create the RAID

mdadm --create --verbose /dev/md0 --level=5 --raid-devices=5 /dev/sda /dev/sdc /dev/sdd /dev/sde /dev/sdf

After the RAID has been completed (watch cat /proc/mdstat), I store the RAID configurations

mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Then I removed some parts of the entry in mdadm.conf. The resulting file looks as follows:

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
#DEVICE partitions containers
#DEVICE partitions

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR root

# definitions of existing MD arrays

# This file was auto-generated on Fri, 14 Mar 2014 23:38:10 +0100
# by mkconf $Id$
ARRAY /dev/md0 UUID=b73a8b66:0681239e:2c1dd406:4907f892

A check if the RAID is working (mdadm --detail /dev/md0) returns the following:

/dev/md0:
Version : 1.2
Creation Time : Sat Apr 19 15:49:03 2014
Raid Level : raid5
Array Size : 7813531648 (7451.56 GiB 8001.06 GB)
Used Dev Size : 1953382912 (1862.89 GiB 2000.26 GB)
Raid Devices : 5
Total Devices : 5
Persistence : Superblock is persistent

Update Time : Sat Apr 19 22:13:37 2014
State : clean
Active Devices : 5
Working Devices : 5
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 512K

Name : roembelHomeserver:0  (local to host roembelHomeserver)
UUID : c29ca6ea:951be1e7:ee0911e9:32b215c8
Events : 67

Number   Major   Minor   RaidDevice State
0       8        0        0      active sync   /dev/sda
1       8       32        1      active sync   /dev/sdc
2       8       48        2      active sync   /dev/sdd
3       8       64        3      active sync   /dev/sde
5       8       80        4      active sync   /dev/sdf

As far as I can tell, this all looks good. In a next step I created the file system:

mke2fs -t ext4 /dev/md0

This results in the following output:

mke2fs 1.42.8 (20-Jun-2013)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=128 blocks, Stripe width=512 blocks
244174848 inodes, 1953382912 blocks
97669145 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
59613 block groups
32768 blocks per group, 32768 fragments per group
4096 inodes per group
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
    4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
    102400000, 214990848, 512000000, 550731776, 644972544, 1934917632

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

Then I changed to fstab by adding the following entry at the end of the file:

/dev/md0    /data    ext4    defaults,nobootwait,no fail     0    2

After mounting the RAID (mount -a) I then could use it, create files, access it from other PCs...

Now comes the problem:
After rebooting the server (reboot now), the RAID does not exist anymore, i.e.
- No /dev/md0
- empty /proc/mdstat (besides the Personalities)
- df -h does not show the raid
- mdadm --assemble --scan does not do anything

Does anyone have any suggestions? Did I do something wrong?

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