Add extra disk as /home

Install the disk and then use:

fdisk -l

to check the new device name, if the new disk is not detected try installing scsitools:

apt-get install scsitools

then run:

rescan-scsi-bus

and issue fdisk -l again, supposing your new disk is /dev/sdb use

fdisk /dev/sdb

to create a new partition, press n, then p for a primary partition then enter 1 sinse this will be the only partition on the drive, when it asks you about the first and last cylinders, just use the defaults.
now, to format the newly created partition use:

mkfs.ext4 /dev/sdb1

when done use:

blkid

to check the new partition's uuid and using that edit the /etc/fstab file andding:

UUID=d70d801e-5246-46e2-a7ed-1a95819fd326 /home ext4 errors=remount-ro 0 1

Now mount the new partition on a temporary location with:

mount /dev/sdb1/mnt/

and copy the contents of the current home to the new partition:

cp -r /home/*/mnt/

when done delete all contents from the current home

rm -rf /home/*

unmount the new partiotion

umount /mnt

and remount it under /home

mount /dev/sdb1/home/