I think the reason Desmond suffered when he got on the freighter was because two things. One, he was exposed to all that radiation. Remember Faraday asked if he was exposed to high amounts? He was in the station for 3 years prior to the Losties blowing the door open plus exposed when he turned the fail safe. The second reason is because Frank did not head back on 325 bearing exactly. They were flying through a storm and he was struggling to keep it steady but couldn't. Being off that little cause him to need the constant. But it seems that anyone can do it as long they follow the right bearings

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