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I had one debian 6 system with two harddisks on raid1 set up via mdadm. built on simple partitions (/dev/sdb3, /dev/sdc3). at least on the old system, everything is tied up via UUIDs.

then i inserted another disk on the system and installed ubuntu 12LTS there. everything was working fine, except the raid. i installed mdadm (didn't add any setting for mdadm or fstab) and rebooted.

after that reboot i couldn't get anything on the screen because of the splash thing (tab, alt+f1, esc, nothing gave me any terminal output... remembering why i hate ubuntu) and it hang there for some time (i couldn't hear anything going on on my disks which are quite noisy), i gave up waiting and rebooted.

i rebooted back into the old debian install, and mdadm informed me that one of the raid disks were absent. tried to --re-add and it complained so i just used --add to add the old disk back. After some time syncing, everything was back to normal.

So, the question is, what am i doing wrong on the new install? why did it even messed up with my raid if i haven't add that information anywhere yet? how do i prevent the automatic behaviour that is screwing things up?

sorry for the vague post, but i'm afraid to boot it now to get more info. i can probably poke at the ubuntu install via the debian one if anyone needs more info such as versions and contents of default conf files.

tl;dr: mdadm raid1 on one box. install new distro (outside the raid) how to make it ignore the raid until i can boot and edit it myself?

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Was it really a RAID0? I don't see how you could possibly have mounted that volume at all if one of the members was missing. RAID0 is a recipe for losing all of your data. It is evil incarnate. You should not be using RAID0 unless you have an extremely well-tested backup system. Since you have neglected to include an details like partition layout, LVM config, RAID config, filesystem layout, I doubt we can help much. My guess is that you make a bad choice doing the Ubuntu install and broke something about your RAID. – Zoredache Dec 1 '12 at 1:06
    
i'm a moron. raid 1. mirrored. the raid is perfectly fine after rebuild. the problem is to understand what is happening when ubuntu with a fresh install of mdadm boot. let me update that and the info you mentioned. thanks. – gcb Dec 1 '12 at 1:16

using boot option raid=noautodetect

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