Is disk’s UUID a good identifier for disk

See the uuidgen program which is part of the e2fsprogs package.

According to this, libuuid is now part of util-linux and the inclusion in e2fsprogs is being phased out. However, on new Ubuntu systems, uuidgen is now in the uuid-runtime package.

To create a uuid and save it in a variable:

uuid=$(uuidgen)

On my Ubuntu system, the alpha characters are output as lower case and on my OS X system, they are output as upper case (thanks to David for pointing this out in a comment).

To switch to all upper case (after generating it as above):

uuid=${uuid^^}

To switch to all lower case:

uuid=${uuid,,}

If, for example, you have two UUIDs and you want to compare them in Bash, ignoring their case, you can do a tolower() style comparison like this:

if [[ ${uuid1,,} == ${uuid2,,} ]]

I would start by seeing what mdadm thinks is going on.

http://linux.die.net/man/8/mdadm -D, --detail Print details of one or more md devices.

should print a lot of information as I recall telling about the state of each raid device.

I believe $sudo mdadm --query --detail should do the trick there.

Danger Zone Below Here

Note: Any action below may have significant consequences. If the other disks have different checksums, then entire raid will have to undergo a rebuild which may take hours and have performance impacts. The best case is there is that there has been no writes since the event and re-adding will take a few seconds. The worst case is the raid will have to reconverge.

If it says the array is degraded, then you can $ sudo mdadm --manage --add /dev/sd??

If it says the raid is offline, then you need to call a data recovery expert.