I sort mail for UPS and it's not a good idea to send fragile mail like external hard drives through UPS. Emphasis is placed on sorting mail fast, not gently. Some people including me throw mail and mailbags sometimes. Also mail is smashed hard to fit as much in 1 bag as possible. They have told us that if you don't work fast enough someone will replace you and I see new people all the time, so they mean business. Also they never ever clean the mail bags, so your mail might have some unnecessary filth on it.
I don't know what other mail companies are like, but I imagine they are similar, because of competition and deadlines. Your best bet is to never ship fragile items unless you have too. And if you have to ship fragile items avoid UPS and read as many reviews about other mail companies as possible.