Install from external hard drive

[Kakiray]

I have a netbook with no CD drive and I'm trying to install Crunchbang Linux on it. I don't have a flash drive so I'm trying to install from my USB external HDD. I know my netbook is capable of booting from the external hard drive as I have gotten it to boot a Windows 7 ISO by just copying the contents of the ISO to the drive but I haven't had the same luck with Linux.
How do I get the Linux ISO to boot from my external HDD?

[wojox]

Boot the computer, and when the splash screen appears, somewhere will appear(usually along the bottom) a line of text that prompts you to press a key for boot options, usually an "f" key. You only have a few seconds to do this before the next screen appears, so if you don't catch it the first time, power down and try again.

[Kakiray]

I've already tried going into boot options and manually trying to select the external to boot from but no luck (that's how I got windows to boot as a test).
I also forgot to mention other things I've tried.
I tried just copying the contents of the ISO to the root of the drive and it didn't work.
I've tried using Unetbootin to make the ISO bootable from my external hard drive.
I've tried formatting the external HDD as FAT32 and marking the partition Active and trying both of the steps above but still didn't work
I've tried burning the ISO to a disk and trying it out on another computer so I'm also sure it isn't corrupted as I also checked the MD5 checksum and it matched the one listed.
Any other ideas?