thompson box freezing and switching off?

my thompson box is now 'freezing' nearly every day!, i have alot of series that i record, but still have 44% free space. often i am watching a programme, it will freeze and then just go off and revert back to my freesat. i press the DTR button, and eventually it will come back on,but it happens alot. could it be i have too many series on record?, is there any way of defragging the hard disk?, could be like a computer, getting too full. any advise greatly appreciated.