I'm guessing it's IDE, otherwise the examples below would be a little different.
If you know what drive it is in the system (primary- / secondary- / -master / -slave), you can find it pretty easily. As root, run "fdisk -l /dev/hdX", replacing the X as follows...
primary-master = a
primary-slave = b
secondary-master = c
secondary-slave = d
So, if it's the primary-slave, the device is /dev/hdb. The fdisk -l command will list all partitions on the drive specified. My output on my Linux-only laptop is...
# fdisk -l /dev/hda
Disk /dev/hda: 60.0 GB, 60011642880 bytes
255 heads, 63 sectors/track, 7296 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/hda1 * 1 16 128488+ 83 Linux
/dev/hda2 17 1321 10482412+ 83 Linux
/dev/hda3 1322 1337 128520 82 Linux swap
/dev/hda4 1338 7296 47865667+ 83 Linux
If it's the only Linux partition on the drive, it will stick out in the "System" column.
To find where it might be automagic'ly mounted, look in the file /etc/fstab. This will list all the partitions Linux knows about on your system. Let's pretend you found a Linux partition at /dev/hdb2 with the fdisk command. Then, you look for the entry in your fstab with that partition. It might look like...
/dev/hdb2 /mnt/storage ext3 defaults 1 2
The first column is the partition, the second is the mount-point. In this fake case, the partition is mounted as /mnt/storage. You could then see what was in that partition with "ls /mnt/storage".
Almost forgot... If you don't find it listed in the fstab, then you will have to create a mount-point and manually mount the partition.
# mkdir /mnt/storage
# mount -t ext3 /dev/hdb4 /mnt/storage
# ls /mnt/storage
Copy over whatever you want to back-up, then unmount the partition with...
# umount /mnt/storage