Create mdraid mirror from an existing Wheezy installation
Throughout this guide,
/dev/sda will be referred to as the old drive and
/dev/sdb will be referred to as the new drive. If possible, I suggest disconnecting all other disks attached to the system while performing this maintenance.
apt-get install mdadm rsync
1. Partition the new drive.
sfdisk -d /dev/sda | sfdisk /dev/sdb sfdisk --change-id /dev/sdb 1 DA # change partition-type to, "0xDA" aka "non-fs data" as "linux raid auto" is deprecated