PLANET NVR-420 - Manual (Page 73)

Once it’s detected, go back to the “USB Backup” page and it should be available
for further configuration
The USB hard disk(s) will be listed in the drop-down menu displaying the
remaining disk space. Make your selection from the drop-down menu if you have
more than one disk connected to the NVR.
Next, select channels, which you would like to back up the recording data from.
Maximum 4 channels can be selected at once
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4-CH H.264 Network Video Recorder with HDMI
User’s Manual of NVR-420