First of all, have you run any storage/HDD tests? Usually, HDD errors are fault with the HDD and not the NVR.
If you do want to replace the NVR and are planning to keep the existing cameras then we would recommend the DS-7604NXI-K1/4P 4-ch NVR, but if you think you might upgrade some of the cameras or add more cameras then we would recommend the DS-7608NI-M2/8P 8-ch NVR.