depending on where the hd is connected at, it will be accessible through a different dev device.
primary channnel master - hda
primary channnel slave - hdb
secondary channnel master - hdc
secondary channnel slave - hdd
then from there you can do a
fdisk -l /dev/<drive>
to see which partition you want.. example would be, /dev/hda and it would show you that you had a linux partition at hda5
then do a
mount /dev/hda5 /mnt/<some dir>
then just copy all the data over to the new drive..