For a start, you could use chimps tools.Chimps tools
But i myself have a question in this regard,
I know chimp HDD tools are pretty cool, but i'm perhaps after a different approach.
And for anyone who is listening....
1. open softmodded xbox
2. attach a power splitter cable as required in the chimp tools.
3. attach the new large hdd to the power splitter.
4. reboot the xbox into exox.
5. hot swap the hdd by removing the ide connector from the origional hdd and connecting to the new.
6. format the new hdd via evox format functions, i'm not sure but you may need to reboot and go through the above process again.
7. ftp into the xbox to gain access to the new hdd, copy over the files required to c and e
8. using evox lock the new hdd.
9. put the xbox back together with the new hdd and boot.
Now am I on the right track, this seems to me to be a sound method.
I could be wrong the worst thing i could do is fry the new hdd, yeah?