You can not simply hook the xbox HDD to the PC and think it will work. There are a few ways to get access to it but those are risky.
Instead get yourself a twisted network cable that is long enough to go between your PC and the xbox. For example, mine is 25m and goes from my sleeping room (with the PC) to my living room (with the xbox).
Got that done? Well ok since you have modded the xbox and probably installed EvoX as dashboard or similar follow these steps.
1. Give your xbox a static IP, something like 192.168.0.5 and give your PC the static IP 192.168.0.1.
2. Download the free trail of FlashFXP and connect to 192.168.0.5, username:xbox, password:xbox.
3. Now you will see everything on the xbox, in case you have upgraded your HDD you can now transfer from the xbox hdd too your pc and vice versa.
If you want to backup a game and only have original HDD you can open the D: drive when a game is in and you will see all the game files.
4. transfer them all to your PC
5. Download a program called Qwix (you can also use Craxtion if you feel like, I prefer Qwix) and create an ISO with the files you transferred.
6. Burn the iso and you will have your backup.
Hope that helps...
