HDD is usually for contents downloaded from Xbox Live.

Lemme show you this comparison list:
1) Modded Xbox can't access Xbox Live unlike unmodded ones
2) Modded Xbox don't have the latest firmware unlike unmodded ones
3) Price for a modded Xbox is way more expensive than an unmodded one
4) Price per game for a modded Xbox is cheaper than of an unmodded one
5) Quality-wise, discs for a modded Xbox don't last long unlike unmodded ones
6) Warranty for modded Xbox is from 3rd party shops, where else unmodded ones are from Microsoft
7) Lifespan(price) for modded Xbox is very short; expect to see a sharp drop on secondhand sale, unlike originals.

Be original, buy original