I recently recorded a concert to my MacBook Pro using Reaper and a Fireface UC.  The project was fairly busy as I was using all 8 ins and 8 outs on the UC and then recording straight to a USB external hard drive.  Does using both USB ports in this way doubly drain the CPU power?  Although I set the buffer size to 512 samples, I was horrified to find little buzzes and glitches had been recorded occasionally throughout the audio files.