you need to edit your /etc/fstab to tell the system were and what it is...
# /etc/fstab: static file system information.
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
/dev/hdb3 / ext3 defaults,errors=remount-ro 0 1
/dev/hdb1 /boot ext3 defaults 0 2
/dev/hdb4 /home ext3 defaults 0 2
/dev/hdb2 none swap sw 0 0
/dev/hdc /media/cdrom0 udf,iso9660 user,noauto 0 0
/dev/fd0 /media/floppy0 auto rw,user,noauto 0 0
/dev/sda1 /disk/sda1 auto defaults 0 2
/dev/sda2 /disk/sda2 auto defaults 0 2
/dev/sda3 /disk/sda3 auto defaults 0 2
/dev/sdb2 /disk/diskb2 auto defaults 0 2
that is the fstab off of one of my Debian boxes. it has multiple HDDs.
as i can only guess the 2nd 320G drive you added is ALSO a sATA drive, it will be labeled sdb1 (i think i have that correct)
hda1, hda2, etc are all part of the same drive just different partitions.
hdb1, hdb2, etc. are part of the slave (secondary) IDE drive.
sda1, sda2, etc... is the 1st SCSI, or in your case sATA drive, so sdb1, sdb2, etc... will be the 2nd sATA/SCSI drive connected to your computer.
i hope that helps a tad.
oh after editing your fstab, or while doing so, you will need to create the mount point as well.
example if you want all 320G in 1 partition you could mount is at: /mnt/disk2 then just create the folder(directory) mkdir /mnt/disk2 and set the permissions to what ever you want.