On a (not unrelated to my last question) note, I am 40% of the way through an ISO download, and do not have enough disk space left on the LiveCD to store it. Can I divert it to my external memory? Thanks.
You are currently downloading to your RAM (memory), which is a pretty limited resource compared to hard disks or other storage, and that's why it's been exhausted.
Depending on how you are downloading you may be able to switch, but it's not easy unless you started off with a command-line tool such as