Okay so I had a thought that if you can make a bootable USB drive why cant you do it with an external hard drive? I was wondering is this possible? Has it been done before? If so how, and how many distros could possibly run off of the same drive? Ive done the research I can but Im at job corps so my internet access is pretty limited.

Yes. You just install the operating system to the external insuring you install its bootloader to the master boot record of the external drive and select to boot from the external in the BIOS on boot, usually the F12 key will give you options.

Depends on the type of drive. It's limited by the number of partitions as generally you need at least one partition for each operating system although you could use a Virtual Machine. Old style scsi disks were limited to 15 partitions, IDE 63 and I think SATA 15 also. You might take a read of the link below which has some useful information:

http://www.justlinux.com/forum/showthread.php?t=152404

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Alright thanks man! Ill try it out!

Most new distro's or newer kernel distro's could be installed to a usb device, flash or hard drive are the same to it for the most part.

If you have a newish computer and you power up with the usb attached you may have an option in the bios for boot order.
If you move the usb up in the order it becomes what bios sees as the first hard drive. Normal distro's would usually install all the normal stuff to the usb drive. Be sure to watch out for grub or other boot loaders. The first boot drive should be on new distro's an SDA (sda) number.

Some older or made for older hardware versions still use the hdx way and it may need some issues but almost all of the top 30 at distrowatch.com can be installed to an external usb drive just as it were any normal hard drive. Remember the system if newish controls what the drive is to bios. The bios then rules on how the installer works for the most part.

If you have any doubts then disable the internal hard drive while testing this. Disable by removing drive power, and not bios option.

I always use a virtual machine for this task. While I can't reboot to the usb in a vm yet, I can install it safely then use it to reboot to a real system.