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I've just bought two 1.5TB disks with the aim of creating a 1.5TB RAID1 array.

In my infinite lack of wisdom (and a lack of space inside the computer - it already had 6 disks), I stuck one in, formatted it to EXT4 and proceeded to copy data all over it from drives that I'm subbing out.

Now I want to create a mdadm RAID1 array with its twin disk. I thought this was possible. Probably involving unmounting and dding the content across but I'm sure I saw this somewhere before...

But I can't find the tutorial I used the last time I was setting up mdadm so I'm scared. I don't want to nuke 800gigs of data.

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Just for the records, another step by step guide: How to Set Up a RAID 1 Under GNU/Linux. – Nikos Alexandris Jul 17 '13 at 22:01
up vote 13 down vote accepted

You're most likely talking about this

"Now use mdadm to create the raid arrays. We mark the first drive (sda) as "missing" so it doesn't wipe out our existing data..."

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Yep. Create a degraded array, copy the data to the array, then add the second disk and wait for it to sync up. – Dan Carley Jul 23 '09 at 13:29
MAGIC! Thank you! – Oli Jul 23 '09 at 14:40
Okay, just read through it twice. Their example is a little more expansive than my setup (I've only 1 partition for starters) but do I have the steps right: 1. Create the mdadm array on the second drive. 2. Copy the data across onto the new drive/array. 3. Add the "old" drive to the array... And then mdadm nukes the first disk and restores the data from the second disk... Sound about right? – Oli Jul 23 '09 at 14:57
@Oli: You've got the basic idea, but you will need to reboot onto the new array before adding the old drive into the array. – Daniel Lawson Mar 8 '11 at 8:27
Woah! This is a really good "cheat" :) I will upvote and "steal" this method as my own. – Mister IT Guru Apr 20 '11 at 7:42

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