[ak217]

What happens to these helium hard drives if you repeatedly cycle the pressure, like by taking the hard drive on an airplane?

[rleigh]

Since the partial pressure of helium at ground level and flight level is essentially the same (≈0) there should be very little change, providing the seals are intact. Since they are very securely sealed to contain the helium, changes to the external pressure, unless extreme, should not have any significant effect.