Is there a way to require USB drives that are connect a Mac OS X to be encrypted?
I found this article, New USB Enforced Encryption Device for Mac OS, and possible solutions:


  It has always been a challenge for IT administrators to find solutions
  and tools compatible with Mac OS X, but now securing the network and
  data Mac users access is also becoming a priority. 
  
  CoSoSys' Endpoint Protector 4 with Enforced Encryption on
  removable storage devices for Mac OS X solves these problems. 
  
  It offers an easy, yet secure way for Mac users to encrypt data copied
  onto USB storage devices. With intuitive 'drag &amp; drop' and copy/paste
  features, all information copied on the device is instantly protected
  by password-based military strength AES 256-bit encryption. The
  Enforced Encryption functionality also incorporates an additional
  security feature that deletes information if an attempt is made to
  tamper with the data, or when the device gets lost or stolen.


 Therefore, stopping users from taking classified information from Macs minis, iMacs, Mac Pros, MacBooks onto portable devices has become an imperative for the companies' data security. Locking down and monitoring proactively the use of USB, CD/DVD, card readers, etc. is possible with Device Control by Endpoint Protector for Mac OS X and can be managed centrally from the intuitive web-based interface.