I'm not familiar with this exact model (so what follows is based on my experience working on the RDR-870 series) but my first thoughts would be to look to see if the HDD is near to capacity, and if so delete titles. You may need to delete quite a lot and then wait a little while for the file system to catch up and report the correct free space. Having done that look in the menu for an 'Optimise HDD' function which is similar to defragmenting a drive on a PC.
If non of that works then I would be looking at the HDD, however swapping them for a new one almost certainly requires the use of a 'Service Remote' to initially format the drive and allow the recorder to recognise it. The service remote also allows you to format the existing drive which may in itself correct the issue.
Also although any normal drive would normally work, some are far more suited to this use than others. Fitting a non optimal HDD can in itself give rise to freezing on playback.
