I have the same problem. I have just created a Sony support request and will let you know the outcome!
I'm very disappointed that I spend a lot of money on this TV and the PVR feature does not appear to work with USB drives. Also note that the PS3 seems to also have the same issue.
I tried all the things that were recommended on other forums. For example ensuring there is only one partition and formatting to FAT32. The USB hard drive also has its own power supply and so power cannot be the issue.