The problem is, with a FAT16 filesystem the maximum size of the partition is 2 GB. So you would be able to use only 2 GB (or, if your DVR is clever and can use multiple partitions, 8 GB of the space on your drive.
That said, you can use Disk Manager (Control Panel -> Administrative Tools -> Computer Management, Disk Manager tab) to delete all partitions on the drive, create a 2 GB partition, and format it as FAT16. All data on the drive will be wiped.
It does not sound like a good idea to me. Does the HDTV officially support this way of saving shows? If so, does the manual say anything about the format of the external drive?