Welcome to LQ. Do you have available, unallocated space on your external drive into which you can install Linux? In other words, if your external drive is 80G, and it contains an 80G NTFS partition, there's no place to put Linux. Even if you only have 10G of data in that 80G partition, all of it has already been allocated.
It would be helpful for you to post the output of the following command, which will list your existing partitions (be sure the external drive is connected)
Note that's a lowercase "L"