Uniden GDVR 20 Series Owner's Manual page 61

No image or having mosaic effect when preview network image
or playback video files
Possible reasons:
Network speed is inconsistent or too slow.
The logged in user has no surveillance authority.
The device has no video output. Camera disconnected.
Something wrong with the USB backup.
Possible reasons:
The CPU is busy please stop recording and then go on backup.
The data volume exceeds the backup storage capacity and it will cause
burning errors
The backup device is not compatible.
The backup device is damaged.
Alerts is not working.
Possible reasons:
The setting of the alert is not correct.
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