I have a new HR24-500 and I want to connect an external sata hard drive. Does this replace the internal hard drive or can you transfer your recordings to it?
It's an either or situation. You cannot transfer recordings to or from the external drive. In fact, you can't move the external to a different DVR and keep the recordings intact.
When you connect an external drive, whatever you have recorded on your internal drive becomes inaccessible. So your strategy of first filling up your internal drive might not be the most convenient. You can go back and watch programs from your internal drive by turning off and unplugging the DVR, disconnecting the external drive, then starting up again, but that's a pain.