busybx umount does not support -O flag, but it's used in netmount script
the /etc/init.d/netmount script contains this code:
if [ “$RC_UNAME” = Linux ] && [ $retval = 0 ]; then
umount -a -O _netdev
retval=$?
fi
it shows an error when shutting down/rebooting system with attached usb
disk, because -O is not supported by busybox
it is a cosmetic issue, hence low priority
(from redmine: issue id 9923, created on 2019-01-26)
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