I guess we are talking different things.
To use USB for data/obb, you need to setup that external storage path on xinternalsd config. If you meant ext partition, it's not necessary. Ext partitions are just for using external storage as an extension of internal sdcard, and it's just really useful for moving apks to external storage on android kitkat or above (because, on jellybean - stock ouya firmware - you can move to usb as you know).
Cheers,
Fabrício.
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