A First Time Installation is a factory reset that clears the operational memory but not any recorded programmes. You will, however, lose any scheduled recording markers for programmes that have not yet been broadcast. So please take a note of any programmes you have scheduled to record in the future before starting the First Time Installation. You will need to re enter them into the library after the First Time Installation.
A Hard Drive Disc Format (HDD) will erase all programmes you have recorded but not any scheduled recordings yet to be broadcast.
Sometimes it is necessary to perform either or both of these functions when you are experiencing issues with your Digital TV Receiver.