Are external hard drives designed to be protected against electrostatic charge?
Being in an enclosure it will have as much protection against ESD as any other enclosed electronics, from your laptop to your DVD player.

The only chance of damage through ESD will be via the USB (or FireWire, Thunderbolt, E-SATA etc) port, but due to the design of these ports, it is pretty much impossible to come into contact with the data pins, making the chance of ESD damage negligible.