There are a lot of myths and just plain bad information out there about EMP and its potential effects.

Most modern electronic equipment is a lot more resistant to the effects of EMP than you might think. Its not a 100% thing though. Some equipment will survive and others not, with no discernible reason why.

Grounding has no effect whatsoever on the protection a Faraday cage provides, nor does it provide any additional protection against EMP.

EMP is as much an antenna issue as anything else. If your equipment is not connected to a long wire, its probably reasonably safe.

Any metal enclosure such as a car body, an aluminum case, or a metal building can provide some protection, but it is not a proper Faraday cage, because it does not completely enclose anything. There are plenty of places for leakage.

Unfortunately, much of the literature on the Internet about EMP is oriented toward military applications where they consider any degradation due to the effects of EMP as undesirable, so they go to extreme lengths to prevent it. This makes it appear as if EMP is some kind of monster that will destroy everything electrical in its path, which is far from the truth.