In case you are repartitioning a drive - the UUID's (Universally Unique Identifier) get changed. This is more and more important as many devices today support hot-plugging or are external. You have several ways to identify the partitions. 2 of these are here. A simple way is the following:

$  ls -l /dev/disk/by-uuid/
 458b1af6-46e4-4980-8dab-ceaa2083459a -> ../../sda3
 6c80c066-08ad-4c22-9366-fe10ff8350e2 -> ../../sda2
 d18ad271-6831-4b83-980f-fe919ff65fd1 -> ../../sda1

will list you all UUID linked to the proper old known devices.
A call as Root of blkid will provide you the UUID of your harddisk.

$ sudo blkid
[sudo] password for jmertin:
/dev/sda1: LABEL="ROOT" UUID="d18ad271-6831-4b83-980f-fe919ff65fd1" SEC_TYPE="ext2" TYPE="ext3"
/dev/sda2: TYPE="swap" UUID="6c80c066-08ad-4c22-9366-fe10ff8350e2"
/dev/sda3: UUID="458b1af6-46e4-4980-8dab-ceaa2083459a" SEC_TYPE="ext2" TYPE="ext3"