"more over, my laptop DVD rom is combo, since it shouldn't be IDE....what is its interface to the laptop?"
It's still an IDE (EIDE) device. Just because it's a combo doesn't mean it's different, it just has the functionality of both drives.
You'll have to check the technical specs on your external harddrive. Just because it's connected with a USB2 cable, doesn't mean it's actually achieving those speeds. Remember, an external harddrive is just a HDD outside the box. Its still grounded by the same connection principles as if it was inside the box. Most external drives have a build-in IDE ATA controller card, which communicates directly to the USB2 interface connector, which then goes to your machine. If your drive is running at ATA133, it doesn't matter that the cable can handle 480mbs, yer only gonna get 133 data rates. Yer pipe is plenty big, but yer not channeling hoover dam.