For the record, the San Diego feed at full bitrate from Cox, will fill my drive with only about 19 hours of content. HBO, on the other hand, will hold about 26 hours of content easily. Starz HD programming will hold even more, depending on content.
You should also consider that the percentage of drive space used is just an estimate based on the closest whole number to the total remaining space of the drive, including the space of the buffers.
As an example, if you calculated your drive space total (100%) divided by space used (34%) = 2.94 and multiply that by your hours used (6), you get your value of 17.7 hours of total estimated space.
In reality, you should be multiplying by the TOTAL amount of hours stored including the 1 hour of buffer time. 2.94 x 7 = 20.5 hours of estimated HD recording space.
That's not to say the 1 hour buffer is included in your total recording time, but if you base your estimated recording time on remaining space available, then it has to be factored in as a variable to the equation.
Confused yet? I have filled my drive many times to see what happens and it does hold roughly 20 hours of HD content, give or take some time, depending on the source material.
edit: if you ever do fill your drive, it will go to 99% and then have buffer problems. Once that happens, you have to delete some content to make space available. Then you MUST power off the box, pull the AC plug for 30 seconds, then while holding the power button, insert the power plug while it reboots and resets the software.