Which size is the smallest HD that I can use to install Snow leopard? I had problems with my iMac (20" 2009 Intel and 4GB RAM) as I was updating the OS from OSX 10.5 to 10.6. It told me that I can't update in my main hard drive, so to don't format it, and my main hard drive had problems and I want to save information from it as I didn't do any backup. I had the originals Snow Leopard install CDs and I was thinking to install OSX 10.6 in a 32GB thumb-drive if that is enough storage space. 

What is the smallest size external HD or FlashDrive that would allow me to install OSX 10.6? I'm hoping to avoid having to purchase a full 500GB external HD.
Snow Leopard requires about 5 GB of space to install itself and will run with not much more space than that total in most cases.

Installing on a low capacity flash drive (USB storage) is possible, but very slow since the chips there are optimized for low power and react far, far slower than even a basic, bargain basement spindle hard drive.