Well guys, good news.
I had no time to backup all my files from my external HDD, so I decided to apply the easiest and fastest method... and it works! ^_^
You don't even have to backup your files or change the dimensions of your partitions. Just right click on Computer and select Manage. Go to Disk Management and set the partition of your external hard disk as active. Next, copy all the files from your Windows 7 disk image into the partition you've set active. An fast way to do that is, if you use WinRAR, open the disk image with it and extract all the files to your external hard drive.
That's all! I don't even had the need to reconfigure the USB emulation on the BIOS. Just use your boot selector when your machine is in the POST, choose your external hard disk from the list and the rest goes like magic. ^_^
Tested on Asus P5N32-E SLI, Acer Aspire 4520, Gigabyte GA-945GCM-S2, Biostar P4M890-M7 and Biostar 945GC-M7 TE. I want to test it later with an ASRock motherboard because this motherboard's brand was always a problem when booting USB devices. :S
Ah! I almost forgot: this method also applies to pen drives. I've tested it with my cheap Kingston DataTraveler 101 with 8 GB. of capacity on the same machines I've mentioned before.
Hope it helps!
Sayonara! ^_^