NSA325 HDD red LED during backup

[johnmb]

I'm have a Zyxel NSA325v2 running latest V4.81 firmware. I have 2 WD 3TB Disks in a RAID1 configuration and a single 3TB disk I backup to via USB. When the backup to the USB disk is running the LED for the second internal disk is solid red while the LED for the first internal disk is solid green. Once the backup is complete the LED for the second disk changes from solid red back to solid green.
The SMART data for the second disk shows Raw_read_error_rate = 969.
The 325 manual states "LED stays red until the NSA restarts" if "NSA detected an error on the hard drive". In my case the LED does not stay red, it changes back to green.
Is there a problem with my second disk?