1. as far as im aware, xbla games/dlc can only be played from the hdd up top, and not off an external hdd. you might want to try extracting the files from the containers, and see if you can launch them from the hdd.
2. search for xbox image browser. this will allow you to extract the files off the disk. i havent tested games on xbreboot 8955_3/3a yet, but with v _1, you had to use X360GameHack to patch the default.xex files(program does it automatically). then from there you can play them off the external hdd, or use quickboot to make a shortcut and store it on your 360 hdd/mu.
3. you could look into modding the original 360 hdd and either put an externally mounted esata or sata port and if your hdd enclosure has esata(i dont think it does), you can run the hdd to the top port, and it will see any size hdd. ive done this with my 2tb drive, but it will see the whole drive, not any partitions, so if you want a larger hdd to show up as a 360 hdd, it will have to be dedicated.
all of this stuff has been posted in the past on here, but i like giving my 2 cents worth.