They can freeze and split - do not leave them in your car in the winter. They are also in plastic cases - do not leave them in your car in summer. The hard drives are susceptible to damage due to excessive or repetitive shock or vibration. Do not use near equipment such as power saws or drills. Do not drop them.
Portable computers are desirable items that are portable .
Take care that they do not get stolen. Never leave them unattended, especially on campus. Do not leave them in your car. Avoid leaving them in hotel rooms.