Likelihood: Geomagnetic storms are 100% likely events that can cause disruption and damage to electronics and power infrastructure. The magnitude has been covered by recent studies by the National Academy of Sciences, NERC, and the EMP Commission. Recent predictions of the impact of large storms such as those of 1921 and the Carrington Event have been recently revised to show that extended power outages can occur with restoration times between 4-10 years.
Protection: Equipment and infrastructure can be protected from the electromagnetic effects of geomagnetic storms, and, if done correctly, can also protect against manmade EMP.
(See the Lawrence Joseph article: "The Solar 'Katrina' Storm that Could Take Out Our Power Grid", July 15, 2010 and the recognition of the work of John Kappenman.