Mac – Install Ubuntu without a CD drive

So here is the situation. I own a older Mac Mini (Intel core duo, 1.66 GHz) and thus will not be able to update to Lion. Also the internal CD drive failed about 3 months ago.

Now I love my Mac. & love "Snow Leopard " but I have been playing around a lot with Ubuntu, running it in a virtual machine. I think that I can replace all or almost all programs that I now use on the Mac. with programs running on Ubuntu.

Since I will not be able to buy new hardware any time soon, and this Mac Mini is now "officially" a low end un-supported machine. I would like to do the following …..

Completely erase the internal hard drive on the Mac Mini.
Yet keep a bootable copy of the internal system on an external firewire hard drive (super duper).
Install and run Ubuntu on the internal drive, retaining the ability to boot into Mac OS 10.6.8 from the external drive.
So that I can retain both work environments until I am working 95% percent of the time in Ubuntu.

Did I say that the internal CD drive failed. Yes, It will not read nor write. Gone. I am not going to replace it as it would cost more than the Mac is worth.. ( apple quoted me $130) so…. I have questions ….

(first about me .. I am not afraid to drop into terminal, have written many apple scripts, an might be called a "super user")

1.) How the bloody hell can I install Ubuntu without a working CD drive?
I have tried to make a bootable thumb drive with the ISO for ubuntu and boot from that. No luck . Will keep trying this as it is probably the best way. But any advice would be helpful.

2.) Once I get Ubuntu up and running on the internal drive will It be difficult to re-boot into the external drive with Mac OS 10.6.8?
This is most likely trivial, but I can not test it so I am A little uneasy.

3.) if I buy an external CD USB drive can I use that to install Ubuntu? (about $10.00 on e-bay)

Thank you for reading…