That can be caused by bad hard drive.
If the NVS2501 can be connected to the computer using USB/Firewire, backup your data and change the internal HDD.
Check here for recommended drives.
Leave your e-mail then we will send you internal drive install guide.
In the worst case, NVS2501 might not be connect to computer.
In that case, remove the internal disk and use USB external drive case and connect to computer to pull out data.