The Pentagon had several terabytes of data related to the F-35 fighter stolen in 2009. This is just one of many hacking incidents involving exfiltration of large amounts of data. Law firms are particularly vulnerable to these kinds of attacks.
If your firm has a network drive with everyone's files or a document management system then you're likely vulnerable to the mass theft of documents. Whenever I read about these kinds of losses I always wonder why the systems allowed thousands or millions of documents to be downloaded at once.
Your file management system should throttle everyday access so more than X number of documents per minute can't be downloaded. Do not allow access to every file on a giant multi-gigabyte network drive. Hackers or angry/malicious employees should never be able to grab all of your files at once.