Can I read from a 2-disk RAID 1 array from a 2-bay external USB disk dock?

Question

I have 2 disks which were the only disks in a Linux RAID 1 array. The RAID information is still readable via mdadm when mounted externally via USB using my 2-bay disk dock. Is there a way I can read data from this array without installing them as internal drives? They were initially created on my QNAP NAS (TS-451+) and all bays of the NAS are occupied. I need to compare data in this array with another RAID 1 array in my NAS.

Answer 1

I was able to pull this together myself.

First step is to find out which device the external USB drives are.

df -h | grep -v qpkg

On a QNAP it's usually at the end of the list showing something like this:

/dev/sdf1                 1.8T      1.8T     58.2G  97% /share/external/DEV3302_1
/dev/sdg2               459.8G    362.0G     97.8G  79% /share/external/DEV3305_2

First column is the device name. Usually it's sdf[1-9] and sdg[1-9]. Then you can use mdadm command to run the RAID group:

mdadm -A -R /dev/md0 /dev/sdf3 /dev/sdg3

I used a non-existing device name md0. I dared not to use an existing one. Last step is to mount the newly started device using mount command:

mount -t ext4 /dev/md0 /mnt/extusbraid1