On ND8301/ND8401, if one of raided HDD went defective, how can I rebuild RAID with new HDD?

Once you replace the faulty drive with a new one, an automatic rebuild will begin, and its progress will be indicated by a status bar as shown below.

The process will take hours depending on the sizes of your disk drives, e.g., 8 hours if using 2TB drives in a RAID5 volume.

If using a RAID5 volume, the recording task will be halted during the rebuild process. Recording will continue if using storage volumes in other RAID levels.

(For more detail you may refer to
http://download.vivotek.com/downloadfile/downloads/usersmanuals/nd8401manual_en.pdf P.198)

Since ND8401 is on Windows Embedded OS, the functionality of RAID is provided by Intel driver, in other words, it would also determine when to rebuild RAID.
If automatic rebuild did not start just in place, you could go to Setup Wizard and try to rebuild RAID manually,