System wont boot from degraded RAID6

I have a Debian Wheezy with 3 Partitions as RAID6. MD0 is for Root and OS, MD1 is for SWAP and MD2 for Files. While copy files to MD2 the system reports kernel panic and halts. I have had shutdown on the hard way. When I boot the system I get the message:

md0: cannot start dirty degraded array
md0: failed to run raid set
failed to run_array md0 inpu/output error
...
md0 is already in use
gave up waiting for root device common problems...
alert! dev/disk/by-uuid/eb8r..... does not exist.
dropping to a shell!
cant access tty job control turned off
((initframs)

proc/mdstat shows for md0 missing Partion sdg2. mdadm detail shows:

Raid Devices: 7
Total Devices: 6
Active D: 6
Working: 6
0...active sync sda2
...
6 removed sdf2

I am unsure what to do.

Solution 1:

Restore from backup.

The next time you build the machine, make md0 RAID1 and only the large data stores RAID6.

Or better, invest in a hardware RAID card.