How to read a RAID1 hard disk if you can’t use the RAID Controller?

December 30, 2007

I had following problem:
An 3ware RAID controller got destroyed after a power surge and I needed to read the data on the RAID without waiting for a replacement controller. I therefore attached one of the hard disks via an USB adapter (PATA/SATA to USB2) to my notebook. The problem now is that a RAID Controller uses some blocks at the beginning of the disk for itself, this leads to the problem that the partition table is not found by the Linux kernel.

Solution:
I used LDE (The Linux Disk Editor) to find out how many blocks are used by the RAID controller. My 3ware controller used 0×200 = 512 blocks with 1024 byte = 524288 bytes offset. I therefore created an loop back device with this offset:

# losetup -o 524288 /dev/loop0 /dev/sdb

and I checked if the partition table is readable via:

# fdisk -l /dev/loop/0 Disk /dev/loop/0: 160.0 GB, 160041361408 bytes 255 heads, 63 sectors/track, 19457 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Â Â Â Start Â Â Â Â End Â Â Â Blocks Â Id Â System /dev/loop/0p1 Â Â Â Â Â Â Â 1 Â Â Â Â 249 Â Â 2000061 Â 82 Â Linux swap / Solaris /dev/loop/0p2 Â * Â Â Â Â 250 Â Â Â Â 1494 Â Â 10000462+ Â 83 Â Linux /dev/loop/0p3 Â Â Â Â Â Â 1495 Â Â Â 19457 Â 144287797+ Â 83 Â Linux

Looks good, but mounting didn’t work directly but I didn’t have the time to look into more detail (maybe udev didn’t created the /dev entries), it was important to get the data of the disks as fast as possible. So I just removed the loop back device with

# losetup -d /dev/loop0

and started to do some math. The swap partition was of no need, I needed only the two ext3 partitions. I calculated the offset of the first of these partitions this way: 249 units with each 8225280 bytes and of cource the RAID controller offset:

249*8225280 + 524288 = 2048619008

with this I did the mounting like this:

# losetup -o 2048619008 /dev/loop0 /dev/sdb # mount -r /dev/loop0 /tmp/1

and for the second ext3 partition this.

1494*8225280 + 524288 = 12289092608

# losetup -o 12289092608 /dev/loop1 /dev/sdb # mount -r /dev/loop1 /tmp/2/

I hope this helps someone in a similar situation as I was.

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