Q: Trying to save the contents of a harddisk in a faulty iMac 21,5"

I own an 21,5 inch late model 2009. This iMac is very unstable. It almost instantly hangs after a short period, no matter what I do. Now I want to save the content of my 500GB hard drive. I have an empty 320GB WD 2,5" flash drive. At this time I do not yet want to open the iMac, remove the harddrive and connect the harddrive to a Windows PC using a docking station. Instead I tried booting the iMac using an 8GB flash drive containing a bootable Ubuntu linux system. I prepared the flash drive using a Windows computer. The problem is, no matter what I do, I can't get the iMac using this flash drive as a boot device. The flash drive is visible when I use the Apple disk/file explore utility. However, the flash drive does not appear in the list of bootable devices. I only have the 500GB internal drive and a network drive (which I don't have).

How do I get the iMac booting from the 8GB flash drive? Do I have to prepare the flash drive using another Apple computer? Besides the iMac I own a MacBook; can I use this computer to prepare the flash drive? I tested the flash drive using a Windows 8.1 computer and this works fine.

Kind regards from the Netherlands