NVIDIA Shield Pro TVs, the ones with a 500GB hard drive, seem to have a problem in some of the hard drives. This is most noticeable to users when they try to perform a system update and instead see the fastboot menu. NVIDIA also says the problem may occur when users see severe pixelation in the recommendations row.
If you have either of these symptoms, you should go to NVIDIA’s support website and apply for a replacement (an RMA). Once that is approved you will receive a replacement unit that does not have a hard drive problem. If you have any other questions, NVIDIA has posted a small FAQ in their forums.