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two days ago one of my two drives in my software RAID 1 (that was setup using gnome-disks) configuration failed. I could not access the files in the meantime. After short research it seems that I have to replace the failed disk and rebuild the RAID to access my files again. So no problem today I got my replacement disk, removed the old broken drive from the raid and tried to add the new drive to the disk.

(Before I copied the partition layout from the working drive with: sfdisk -d /dev/sdb | sfdisk /dev/sda)

When I try to add a new disk to my RAID (that shows up in my gnome-disks) with mdadm —manage /dev/md0 —add /dev/sda, I run into this error:

mdadm: Cannot get array info for /dev/md0

The same thing happens to basically any operation using mdadm on my /dev/md0.

How can I get my RAID back working?

Some info:

Ubuntu 15.04 (64bit)

mdadm --version

mdadm - v3.3 - 3rd September 2013

uname -a

Linux Z97X 3.19.0-27-generic #29-Ubuntu SMP Fri Aug 14 21:43:37 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

cat /proc/mdstat

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md0 : inactive sdb[0](S)
      976627383 blocks super 1.2

unused devices: <none>

mdadm -D /dev/md0
/dev/md0:
        Version : 1.2
     Raid Level : raid0
  Total Devices : 1
    Persistence : Superblock is persistent

          State : inactive

           Name : PC:0
           UUID : XXXXXXXX:7af47836:XXXXXXX:d7360235
         Events : 3569

    Number   Major   Minor   RaidDevice

       -       8       16        -        /dev/sdb

Why is Raid Level : raid0 when I setup this as raid1 in gnome-disks / it's shown as raid1 in gnome disks?

And one question in general: My understanding when I put up my RAID 1 was that if one disk fails I can still access my data because data is mirrored. But when the disk failed this was obviously not the case. Why? Is my understanding of RAID 1 wrong?

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