As a storm passes, air pressure diminishes ( Hurricane Pressure Profile link).
This happens to a much greater degree as the eye of a hurricane passes. The lower pressure causes air to be withdrawn
from the inside of hard drives (there is a tiny vent for pressure equalization). When the storm exits, and normal pressure
resumes, ambient air is pulled into the "sealed" portion of the drive. Immediately after the storm, this air is laden with moisture
and potentially dissolved salt. Both can cause a drive to fail and data to be permanently lost. Salt is especially damaging as it
crystallizes onto the surface of the data platter, basically forming sandpaper. A modern hard drive's heads "fly" at a height of
3 nanometers above the platter (closer than the thickness of a fingerprint).

To prevent damage, put your computer and external hard drives into plastic garbage bags. Don't blow them up, just a little clearance is fine.
Seal the bag until after the storm has passed and the air pressure is normal. The drives will have pulled in the dry air from the bag.

Remember to do this for all the devices that might be exposed that have conventional hard drives, such as set top boxes with program storage,
as well as your computers and external hard drives, or any loose drives you have stored.