if I remember correctly, the small "backup" partition holds a backup of the drives system software and should be left well alone - its for recovering the drive if it goes wrong, and holds the software images for the setup utilities.
You should be trying to use the rest of the drive. You say you have TWO other partitions? Then presumably your device has TWO internal hard drives which can be used either as a RAID or as individual devices? If you can't write to them, I'd hazard a guess that the internal RAID has broken and needs to be recreated. That varies from model to model, but usually theres some kind of raid selection switch or button on the device, following which you have to reformat
Have you still got the instructions for it?