Now I am more clear how you are using the xbox
I have 3 options for you.
First, if you want to maintain the xbox drive structure, you need to reinitialize your xbox hard-disk using homebrew tool such as slayer. The tool will recreate the entire C+E+F+G+X+Y+Z partitions. Then start the whole installation process again.
Second, if you dont care about F drive and you just want to recover the missing 120Gb.
1. Find the boundary of hda54 (assume x)
2. Find the beginning of hda56 (assume y)
3. Create a hda3 in start (x+1), end (y-1) location
4. Format hda3 as iserfs
5. Mount it as one of your partition
Lastly, if you want to convert the entire hd as native iserfs, this what you do. You need to maintain minimum the xbox E partition (hda50) to store the gentoox.xbe and initrd.gz. Basically you follow the same routine as I posted, but you delete away the unwanted F+G+X+Y+Z partitions
1. revert booting mode to original loopback e:\rootfs (replace initrd.gz with oldinitrd.gz)
2. find the boundary (assume x) of hda50
3. delete hda51,52,53,54,55,56 (see the example script)
4. create hda1 at x+1 for 256M.
5. At this point, you should have about 155Gb free space (E drive is 5Gb).
5. create as hda2 after hda1. This hda2 can now take all the 155 Gb, or you can split out as many partitions as you want (again see the example script).
6. Follow the example of the tutorial.