Adding a new disk to an existing computer, and make it RAID-1 ?

Hello everyone, I think it's been four years since I last came here to find information, time sure flies ! At that time, I wanted to fix my home desktop using Mandriva, and, four years later, I want to tweak my brand new dedicated web server, haha ^^ Thank you to all the kind linux talented persons, this community is great ! I'm moving on to the question I'm asking myself, in short : please, is it possible without extreme complications to add a new disk to a non-RAID OS, and make it all become RAID-1 ?:) To give you more background information : - I have a dedicated server at a web host (OVH, Kimsufi 16G, if you wonder), this is a gentoo OS, with good specs, but only one hard disk. - the server is already up and running, all set up correctly (knocking wood, I'm a newbie at all that stuff) - my web host offers to add a new USB hard disk to my dedi for a very low price, this disk would have exactly the same size as the dedi's disk (2 TB). - I want to benefit the increased security offered by Raid-1, and turn my non-RAID operating system into a RAID-1 OS. Nothing beats the source usually, but it's a bit too complex for me, here is Gentoo's wiki page on hardware RAID : RAID/Software - Gentoo Linux Wiki Most of it doesn't concern me, it tells about setting up the operating system, while my dedi came up as already installed. Let's scroll down, there's this passage precisely : Quote: Note: If for some crazy reason (like if you're migrating from a single disk to two...) you only have one of the hard drives on hand and want to setup a mirror (RAID-1) you can specify 'missing' instead of the second device. Then when you install the second device you can add it to the array and it will sync automatically. There may be weirdness when adding another hard drive to the system when it comes to configuring the boot loader (due to drive order/numbering). Note: For the situation above, adding a disk later just worked with the command "mdadm /dev/mdX --add /dev/sdY" where X and Y are case specific, of course. When doing the standard procedure, partitioning the new disk and doing "mdadm /dev/mdX --add /dev/sdYZ" - where Z is the partition - I ended in "mdadm: /dev/sdYZ not large enough to join array" error. Have no idea why is this, as the disks (in my case) were identical (same maker, size and model)" However, if I understood right, these two "note" are still for an OS on which there was mdam and the RAID configuration right from the start, right ? Can I still install RAID options, utilities, driver, on a non-RAID filesystem with just one hard disk, and then, later, proceed to making it become RAID-1 with a freshly plugged hard disk ? As you can see, I'm confused and lose, I apologize :-/ Thank you VERY MUCH if you can clarify this to me, I'll be very grateful ! :) Sabin

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