i m using windows 7(64-bit). I have external hdd-100GB. But it doesn't show up in my windows. I have tried it in another pc(& in my desktop before i installed this windows) and it works fine, so it must be a problem with my windows. I have changed from IDE mode to AHCI mode to check but it's still same. It isn't working. It doesn't show up in Device manager or anywhere , windows simply doesn't catch it. I have tried different usb ports.
Your cd-rom might actually use SATA cable (red thin cable) instead of IDE cable (usually white and wide cable) so if you set yours from AHCI (Sata) to IDE, most SATA hardware will not work...
Usually setting up ACHI or IDE just to detect an external HDD with USB port seems unlikely to happen because external can run in both mode without any problem.
To confirms things up, you can try with another external (if you had another one) and test it on your computer. If it can't detect yours, then that means you have a missing driver that can let external to run in AHCI mode.