What do EMP's damage?

Okay i have a pretty good knowledge of EMP's but im still not clear on what things they damage, ive been under the impression if an electronic device is turned off but im not sure if thats true or not. And whether something liek that would erase a flash drive or harddrive even if turned off. Any anwsers to these deep questions would be appreciated.

www.airpower.maxwell.af.mil...

This link will explain everything you need to know.