There is a particularity standard with a developed body of law in the search warrant area that now appears to be imported into the test for assertion of the act of production Fifth Amendment objection to compulsory testimony. Bottom line, that test will guard against unfocused fishing expeditions.

This is interesting but its about as clear as mud. If the police can subpoena a whole diary they know you have what is to say they cannot subpoena a whole hard drive they know you have? What is "reasonably particular?"

Also, say for instance, that through an administrative subpoena the police obtain noncontent information about emails - the dates, to, and from information. They can then subpoena the contents, without a warrant, if they think you've got those contents on your hard drive. The specific emails ought to be "reasonbly particular" enough.