By the expression "to a fro" we can infer that the raven was going back and forth from the ark. The Hebrew verbs to which this preposition phrase is converted in English mean "going out and returning." This implies that the raven came back to rest on the ark after finding no place to land elsewhere--but apparently the raven never returned inside the ark because it says the raven continued to go to and fro until the waters were dried up.
Ravens can fly for quite awhile, but they are not the kind of bird that can fly continuously, without ever stopping for rest or sleep, as perhaps a storm petrel, swift, or albatross might. Ravens like to rest on some high point where they have a view--and Noah likely counted on this fact when he sent the raven out, knowing that if there were not some place to rest, the raven would return.
I would submit, therefore, that while the raven is not said to have returned to Noah, nor to have re-entered the ark, it did come back to rest on the ark itself until it found rest elsewhere.