2.5" or 3.5" portable? If it's one of the 2.5" ones and has either one of the compatible scorpio series drives inside or one of the av-25 series, then hddhackr can be used on it to make it work the same as a normal xbox hdd would.
You lose the portability though as it would need removing from the enclosure, you'd also lose any data that is on it now as hddhackr will wipe the partitions and set it up for you and then the xbox needs to format it. You're also limited in size as you have to stick to official sizes, so for example ,if you use a 120GB signature file on a 160GB, the xbox will only see 120GB.