Although the allowed scope of a Rule 2004 exam is broad, there are limitations to a court's subpoena power. Rule 45(c)(3)(A)
Federal Rule of Civil Procedure 45(c)(3):
(3) Quashing or Modifying a Subpoena.
(A) When Required. On timely motion, the issuing court must quash or modify a subpoena that:
(i) fails to allow a reasonable time to comply;
(ii) requires a person who is neither a party nor a party's officer to travel more than 100 miles from where that person resides, is employed, or regularly transacts business in person-except that, subject to Rule 45(c)(3)(B)(iii), the person may be commanded to attend a trial by traveling from any such place within the state where the trial is held;
(iii) requires disclosure of privileged or other protected matter, if no exception or waiver applies; or
(iv) subjects a person to undue burden.
(B) When Permitted. To protect a person subject to or affected by a subpoena, the issuing court may, on motion, quash or modify the subpoena if it requires:
(i) disclosing a trade secret or other confidential research, development, or commercial information;
(ii) disclosing an unretained expert's opinion or information that does not describe specific occurrences in dispute and results from the expert's study that was not requested by a party; or
(iii) a person who is neither a party nor a party's officer to incur substantial expense to travel more than 100 miles to attend trial.