Are unpowered SSDs vulnerable to an EMP shock?

The traditional Hard Disk is actually more likely to survive an EMP than an SSD:

Per Wikipedia:

The major effect of a large EMP is to induce high currents and voltages in electrical systems, damaging them or disrupting their function. NEMP weapons are designed to maximise such effects, and are capable of destroying susceptible electronic equipment over a wide area. An indirect effect can be electrical fires caused by the high level of overload.

An SSD is purely an electrical system, so these pulses will easily damage the transistors where an HDD would require a strong magnetic field to physically change the pits of the surface of the disk.

I do not believe either is especially difficult to kill with a good EMP, but I would expect the tolerance of the SSD to be lower than an HHD from an EMP.

As others have mentioned, your SSD is susceptible to EMP while under power. unfourtunately, it looks like it is vulnerable, in many cases, while unplugged as well.

per wikipedia, many types of drives use either a battery or Super-capacitor to prevent data corruption when power is lost. This means that most of the time, an SSD will still carry a charge and is susceptible to EMP. if the drive uses a super-capacitor, and has been disconnected long enough to have dissipated its charge, it should be safe however, unless the EMP itself is sufficiently powerful to destroy the media alone (in which case, I would expect the humans of planet earth to fare far worse than the unplugged SSDs.)

http://en.wikipedia.org/wiki/Solid-state_drive#Battery_or_super_capacitor