The external capsule like the internal capsule is a band of myelinated fibers projecting between the cortex and subcortical structures.
However, unlike the internal capsule it's very thin and lesions produce minimal effects and therefore, it's not commonly tested in USMLE.
However, if you insist have a look at this image
Copyright Note: this is a public domain image from Wikipedia
It's located between the globus pallidus and the claustrum