Unetbootin used to support this, but it has since been removed. This means there is NO way to do this using Unetbootin. However, if all you want to do is test Linux systems without needing to work with partitions, I would recommend looking at a Virtualization system. Something like Virtual Box will allow you to not only test, but install full Linux systems within your Windows system. Other then that your best bet is using Linux distros with Live USB systems, and installing the full systems using the Live USB install.
I have found a system that will work for what you want to do, well, in theory anyway. I don't have an extra hard drive laying around to test it with... But this method should allow you to boot multiple ISOs from your external hard drive.
You will need to use YUMI (Your Universal Multiboot Integrator) to perform this task. This tool is put together by the friendly creator of PendriveLinux.com I have used it before, and to get what you need from it simply: plug in your external hard drive, start the program, select the "Show All Devices" tick box, select your drive, Tell it the distro you are using, navigate to it, and click create.
As stated earlier when this process completes you will be asked if you want to add another iso/distro. Whenever you want to add another after this initial run, all you have to do is start the program and follow the above instructions. And if you ever need to delete an installed system, simply click the "View or remove installed distros".
As per the comments, it looks like Lili USB creator also works. However I have never tried it. So I cannot walk you through the steps.
Hope this helps!