You do not have to format your existing harddrive with ext3 to use with slug. You can make Debian read it for you. By this way you can use the external drive when you disconnect from the slug in any windows machine easily.
I prefer to use ntfs-3g for this purpose. There are other packages for this purpose but I believe this is the best. To install write
apt-get install ntfs-3g
To see the list of connected drives you can use tree.
apt-get install tree
You should see your NTFS formatted drive in the list by typing
You should mount the drive to use it. You can use
mount -t ntfs-3g /dev/sdb1 /mnt/hdd -o force
mount -t ntfs-3g /dev/sdb1 /mnt/hdd
to mount it. You will need to change sdb1 according to the screen you get with tree. Also you should have a directory created /mnt/hdd to mount. Write
to create that directory. Good luck.