interesting point from an MS Q&A back in 2011, quite relevant nowadays:
Q: Can Microsoft guarantee that EU-stored data, held in EU based datacenters, will not leave the European Economic Area under any circumstances — even under a request by the Patriot Act? A: Frazer explained that, as Microsoft is a U.S.-headquartered company, it has to comply with local laws (the United States, as well as any other location where one of its subsidiary companies is based). Though he said that “customers would be informed wherever possible,” he could not provide a guarantee that they would be informed — if a gagging order, injunction or U.S. National Security Letter permits it. He said: “Microsoft cannot provide those guarantees. Neither can any other company.” While it has been suspected for some time, this is the first time Microsoft, or any other company, has given this answer. Any data which is housed, stored or processed by a company, which is a U.S. based company or is wholly owned by a U.S. parent company, is vulnerable to interception and inspection by U.S. authorities.