There are several requirements of hard disk drive (HDD)
1. The disk had better be the one used for survilliance, not for computer use.
Here is reason: http://www.hviewsmart.com/no16-please-don-t-install-desktop-hard-drive-into-dvr-nvr--b12.html
2. The type should be SATA. SATA2(3GB/s) or SATA3（6GB/s）is recommened. If you connect a USB stick or drive to USB port of DVR, DVR can back up video from internal hard disk drive to USB stick, but can't record to USB stick directly.
3. We recommend 5400 RPM.
4. size is 3.5” .
5. The brand can be West digital or Seagate .
6. The capacity should be less than or equal to recorder's maximum capacity.
Here is some models we recommend. But please notice it is just an advice.